diff --git a/src/ZB.MOM.WW.MxGateway.Worker.Tests/MxAccess/MxAccessEventMapperTests.cs b/src/ZB.MOM.WW.MxGateway.Worker.Tests/MxAccess/MxAccessEventMapperTests.cs
index fba52ec..1194d0f 100644
--- a/src/ZB.MOM.WW.MxGateway.Worker.Tests/MxAccess/MxAccessEventMapperTests.cs
+++ b/src/ZB.MOM.WW.MxGateway.Worker.Tests/MxAccess/MxAccessEventMapperTests.cs
@@ -303,6 +303,55 @@ public sealed class MxAccessEventMapperTests
         Assert.Equal(DateTime.SpecifyKind(localWall, DateTimeKind.Local).ToUniversalTime(), utc);
     }
 
+    /// <summary>
+    ///     Verifies format derivation survives a culture whose long-time pattern
+    ///     hides an "ss" inside quoted literal text — the case where blindly
+    ///     inserting ".fff" at the first "ss" produces a format that mangles the
+    ///     value or is outright malformed, and a malformed format makes exact
+    ///     parsing throw rather than report failure. Derivation must reject such
+    ///     a candidate at build time, so parsing here stays exception-free and
+    ///     the culture's own published shape still reads as local wall-clock
+    ///     time. Uses a culture no other test touches, because the format cache
+    ///     compares cultures by name.
+    /// </summary>
+    [Fact]
+    public void TryParseSourceTimestamp_WithLiteralSecondsInCulturePattern_ParsesWithoutThrowing()
+    {
+        CultureInfo pathological = new("en-ZA")
+        {
+            DateTimeFormat =
+            {
+                ShortDatePattern = "M/d/yyyy",
+                LongTimePattern = "'sss' HH:mm:ss",
+            },
+        };
+
+        CultureInfo original = CultureInfo.CurrentCulture;
+        try
+        {
+            CultureInfo.CurrentCulture = pathological;
+            DateTime localWall = new(2026, 3, 26, 13, 38, 22, DateTimeKind.Unspecified);
+
+            // The culture's published shape: "3/26/2026 sss 13:38:22".
+            string published = localWall.ToString(
+                pathological.DateTimeFormat.ShortDatePattern + " " + pathological.DateTimeFormat.LongTimePattern,
+                pathological);
+
+            Assert.True(MxAccessEventMapper.TryParseSourceTimestamp(published, out DateTime utc));
+            Assert.Equal(DateTimeKind.Utc, utc.Kind);
+            Assert.Equal(DateTime.SpecifyKind(localWall, DateTimeKind.Local).ToUniversalTime(), utc);
+
+            // Shapes this culture cannot describe must still fail quietly rather
+            // than throw out of the derived formats.
+            Assert.False(MxAccessEventMapper.TryParseSourceTimestamp("not a timestamp", out _));
+            Assert.False(MxAccessEventMapper.TryParseSourceTimestamp("sss sss sss", out _));
+        }
+        finally
+        {
+            CultureInfo.CurrentCulture = original;
+        }
+    }
+
     /// <summary>
     ///     The parse chain exactly as it stood before PERF-20 added the
     ///     exact-format stage, used as the parity oracle above.
diff --git a/src/ZB.MOM.WW.MxGateway.Worker/MxAccess/MxAccessEventMapper.cs b/src/ZB.MOM.WW.MxGateway.Worker/MxAccess/MxAccessEventMapper.cs
index d1958cf..a045bfb 100644
--- a/src/ZB.MOM.WW.MxGateway.Worker/MxAccess/MxAccessEventMapper.cs
+++ b/src/ZB.MOM.WW.MxGateway.Worker/MxAccess/MxAccessEventMapper.cs
@@ -1,5 +1,6 @@
 using System;
 using System.Collections.Concurrent;
+using System.Collections.Generic;
 using System.Globalization;
 using Google.Protobuf.WellKnownTypes;
 using ZB.MOM.WW.MxGateway.Contracts.Proto;
@@ -15,7 +16,13 @@ public sealed class MxAccessEventMapper
     ///     <see cref="TryParseSourceTimestamp"/> tries before the general parse
     ///     chain. Keyed by <see cref="CultureInfo"/> because the format set is
     ///     derived from the culture's own patterns and the worker host's culture
-    ///     is fixed in practice, so the derivation runs once.
+    ///     is fixed in practice, so the derivation runs once. The cache assumes
+    ///     <see cref="DateTimeFormatInfo"/> patterns are not customized or
+    ///     mutated at runtime — <see cref="CultureInfo"/> compares by name, so a
+    ///     culture carrying custom patterns is served the formats derived for its
+    ///     stock namesake. That direction is safe: a stale format simply stops
+    ///     matching, and the unchanged general chain below still parses the
+    ///     string.
     /// </summary>
     private static readonly ConcurrentDictionary<CultureInfo, string[]> TimestampFormatCache = new();
 
@@ -397,8 +404,12 @@ public sealed class MxAccessEventMapper
 
         const DateTimeStyles styles = DateTimeStyles.AssumeLocal | DateTimeStyles.AdjustToUniversal;
         CultureInfo culture = CultureInfo.CurrentCulture;
+
+        // Empty when the culture's patterns yielded no format that survived
+        // validation; that culture just gets no fast path.
         string[] formats = TimestampFormatCache.GetOrAdd(culture, TimestampFormatFactory);
-        if (DateTime.TryParseExact(text, formats, culture, styles, out DateTime parsed)
+        DateTime parsed;
+        if ((formats.Length > 0 && DateTime.TryParseExact(text, formats, culture, styles, out parsed))
             || DateTime.TryParse(text, culture, styles, out parsed)
             || DateTime.TryParse(text, CultureInfo.InvariantCulture, styles, out parsed))
         {
@@ -419,25 +430,78 @@ public sealed class MxAccessEventMapper
     ///     <c>.fff</c> after the seconds specifier; a pattern without a
     ///     two-digit seconds specifier, or one that already carries a fraction,
     ///     gets only the published pattern.
+    ///     <para>
+    ///         Both candidates are validated by round-tripping a probe instant
+    ///         before they are handed to the event path, because the derivation
+    ///         reads patterns it does not control: the first <c>ss</c> in a
+    ///         pattern could in principle sit inside quoted literal text, and
+    ///         inserting into it would produce a format that either mangles the
+    ///         value or is outright malformed — and a malformed format makes
+    ///         <see cref="DateTime.ParseExact(string, string, IFormatProvider)"/>
+    ///         and its siblings throw <see cref="FormatException"/> rather than
+    ///         report failure. Validation happens here, once per culture, so the
+    ///         event path never carries that risk; a candidate that does not
+    ///         round-trip is dropped, and a culture that loses both simply gets
+    ///         no fast path.
+    ///     </para>
     /// </summary>
     /// <param name="culture">Culture whose patterns the formats are derived from.</param>
-    /// <returns>The formats to try, most likely first.</returns>
+    /// <returns>The validated formats to try, most likely first; empty when none survived.</returns>
     private static string[] BuildTimestampFormats(CultureInfo culture)
     {
         DateTimeFormatInfo formatInfo = culture.DateTimeFormat;
         string shortDate = formatInfo.ShortDatePattern;
         string longTime = formatInfo.LongTimePattern;
+        List<string> formats = new(2);
+
         int secondsIndex = longTime.IndexOf("ss", StringComparison.Ordinal);
         if (secondsIndex >= 0 && longTime.IndexOf('f') < 0 && longTime.IndexOf('F') < 0)
         {
-            return
-            [
-                shortDate + " " + longTime.Insert(secondsIndex + 2, ".fff"),
-                shortDate + " " + longTime,
-            ];
+            string withMilliseconds = shortDate + " " + longTime.Insert(secondsIndex + 2, ".fff");
+            if (RoundTripsProbe(withMilliseconds, culture, new DateTime(2026, 3, 26, 13, 38, 22, 907)))
+            {
+                formats.Add(withMilliseconds);
+            }
         }
 
-        return [shortDate + " " + longTime];
+        string published = shortDate + " " + longTime;
+        if (RoundTripsProbe(published, culture, new DateTime(2026, 3, 26, 13, 38, 22)))
+        {
+            formats.Add(published);
+        }
+
+        return formats.ToArray();
+    }
+
+    /// <summary>
+    ///     Reports whether a derived format both formats and re-parses a probe
+    ///     instant back to itself under the given culture. A format that throws,
+    ///     fails to parse its own output, or loses part of the instant is
+    ///     rejected — the fast path may only ever be a shortcut.
+    /// </summary>
+    /// <param name="format">Candidate format to validate.</param>
+    /// <param name="culture">Culture the format was derived from.</param>
+    /// <param name="probe">Instant to round-trip; carries the precision the format must preserve.</param>
+    /// <returns><see langword="true"/> when the format is safe to use on the event path.</returns>
+    private static bool RoundTripsProbe(
+        string format,
+        CultureInfo culture,
+        DateTime probe)
+    {
+        try
+        {
+            string probeText = probe.ToString(format, culture);
+            return DateTime.TryParseExact(probeText, format, culture, DateTimeStyles.None, out DateTime roundTripped)
+                && roundTripped == probe;
+        }
+        catch (FormatException)
+        {
+            return false;
+        }
+        catch (ArgumentException)
+        {
+            return false;
+        }
     }
 
     private MxArray ConvertBufferedArray(