diff --git a/RunCommand.Test/RunCommandTests.cs b/RunCommand.Test/RunCommandTests.cs
index 767cfbc..64562b3 100644
--- a/RunCommand.Test/RunCommandTests.cs
+++ b/RunCommand.Test/RunCommandTests.cs
@@ -1178,6 +1178,56 @@ public async Task ExecuteAsyncShouldDeliverOutputStillInThePipeWhenTheProcessExi
 		}
 	}
 
+	[TestMethod]
+	public async Task ExecuteAsyncShouldNeverRunTheStandardOutputAndStandardErrorDelegatesAtOnce()
+	{
+		if (RuntimeInformation.IsOSPlatform(OSPlatform.Windows))
+		{
+			Assert.Inconclusive("Needs a shell that interleaves many lines across both streams. The delivery this covers is in platform independent code, so the other legs cover it.");
+		}
+
+		// Both delegates add to one List<T>, which is not thread safe, exactly as a caller capturing
+		// a command's whole output would. Run concurrently, overlapping adds overwrite each other's
+		// slots and lines vanish without an exception.
+		const int linesPerStream = 5000;
+
+		// Whether the delegates overlap depends on how the two reads are scheduled, so one clean
+		// run proves little. Repeat it.
+		for (int attempt = 0; attempt < 5; attempt++)
+		{
+			List<string> lines = [];
+			int active = 0;
+			int overlaps = 0;
+
+			void Add(string line)
+			{
+				if (Interlocked.Increment(ref active) > 1)
+				{
+					_ = Interlocked.Increment(ref overlaps);
+				}
+
+				lines.Add(line);
+				Thread.SpinWait(50);
+				_ = Interlocked.Decrement(ref active);
+			}
+
+			int exitCode = await RunCommand.ExecuteAsync(
+				"sh",
+				["-c", $"i=1; while [ $i -le {linesPerStream} ]; do echo out$i; echo err$i >&2; i=$((i+1)); done"],
+				new LineOutputHandler(Add, Add)).ConfigureAwait(false);
+
+			Assert.AreEqual(0, exitCode);
+			Assert.AreEqual(0, overlaps, $"The delegates ran inside each other {overlaps} times on attempt {attempt + 1}.");
+			Assert.HasCount(linesPerStream * 2, lines, $"Lines were lost on attempt {attempt + 1}.");
+
+			// Serialising delivery must not reorder either stream.
+			string[] expectedOut = [.. Enumerable.Range(1, linesPerStream).Select(i => $"out{i}")];
+			string[] expectedErr = [.. Enumerable.Range(1, linesPerStream).Select(i => $"err{i}")];
+			Assert.AreSequenceEqual(expectedOut, [.. lines.Where(l => l.StartsWith("out", StringComparison.Ordinal))]);
+			Assert.AreSequenceEqual(expectedErr, [.. lines.Where(l => l.StartsWith("err", StringComparison.Ordinal))]);
+		}
+	}
+
 	[TestMethod]
 	[DoNotParallelize]
 	public async Task ExecuteAsyncShouldNotSpinWhileACommandThatClosedItsOutputKeepsRunning()
diff --git a/RunCommand/AsyncProcessStreamReader.cs b/RunCommand/AsyncProcessStreamReader.cs
index 9f0c80e..39fced5 100644
--- a/RunCommand/AsyncProcessStreamReader.cs
+++ b/RunCommand/AsyncProcessStreamReader.cs
@@ -15,6 +15,11 @@ internal sealed class AsyncProcessStreamReader(Process process, OutputHandler ou
 	private bool outputHasEmitted;
 	private bool errorHasEmitted;
 
+	// The two pipes are read concurrently, but their chunks are handed over one at a time. Without
+	// this the standard output and standard error delegates ran inside each other on two thread-pool
+	// threads, and a caller sharing one List<T> or StringBuilder between them lost lines silently.
+	private readonly Lock deliveryLock = new();
+
 	// Read the raw pipes with the caller's encoding rather than through
 	// process.StandardOutput/StandardError. Process builds those StreamReaders with byte-order-mark
 	// detection switched on, which replaces the encoding the caller asked for whenever the output
@@ -152,9 +157,12 @@ private async Task ReadToEnd(StreamReader streamReader, char[] buffer, Action<st
 			string data = new(buffer, 0, charsRead);
 			data = StripLeadingByteOrderMark(data, isStandardOutput);
 
-			if (data.Length > 0)
+			if (data.Length > 0 && onData is not null)
 			{
-				onData?.Invoke(data);
+				lock (deliveryLock)
+				{
+					onData(data);
+				}
 			}
 		}
 	}
diff --git a/RunCommand/OutputHandler.cs b/RunCommand/OutputHandler.cs
index 6d75258..647835e 100644
--- a/RunCommand/OutputHandler.cs
+++ b/RunCommand/OutputHandler.cs
@@ -8,6 +8,11 @@ namespace ktsu.RunCommand;
 /// Handles the output from a command line process, processing both standard output and standard error streams.
 /// It processes the data in undelimited chunks, calling the specified actions for each chunk received.
 /// </summary>
+/// <remarks>
+/// The two streams are read concurrently, but the actions are never run at the same time: each one
+/// returns before the other is called, so both may share state, such as one list collecting all of a
+/// command's output, without locking. Each stream's chunks arrive in the order they were written.
+/// </remarks>
 public class OutputHandler
 {
 	/// <summary>