It seems I have found a "bug" in Mercurial. It takes a shortcut when checking for differences in tracked files. If the file's size and modification time are unchanged, it assumes its contents are unchanged:

$ hg init .
$ cp -p .sccs2hg/2005-06-05_00\:00\:00\,nicstat.c nicstat.c
$ ls -ogE nicstat.c
-rw-r--r--   1   14722 2012-08-24 11:22:48.819451726 -0700 nicstat.c
$ hg add nicstat.c
$ hg commit -m "added nicstat.c"
$ cp -p .sccs2hg/2005-07-02_00\:00\:00\,nicstat.c nicstat.c
$ ls -ogE nicstat.c
-rw-r--r--   1   14722 2012-08-24 11:22:48.819451726 -0700 nicstat.c
$ hg diff
$ hg commit
nothing changed
$ touch nicstat.c
$ hg diff
diff -r b49cf59d431d nicstat.c
--- a/nicstat.c	Fri Aug 24 11:21:27 2012 -0700
+++ b/nicstat.c	Fri Aug 24 11:22:50 2012 -0700
@@ -2,7 +2,7 @@
  * nicstat - print network traffic, Kb/s read and written. Solaris 8+.
  *	"netstat -i" only gives a packet count, this program gives Kbytes.
  *
- * 05-Jun-2005, ver 0.81  (check for new versions, http://www.brendangregg.com)
+ * 02-Jul-2005, ver 0.90  (check for new versions, http://www.brendangregg.com)
  *

[...]

Now, before you agree or disagree with me on whether this is a bug, I will also say that I believe it is a feature. Yes, I feel it is an acceptable shortcut because in "real" situations an edit to a file will change the modification time by at least one second (the resolution that hg diff or hg commit is looking for). The benefit of the shortcut is greatly improved performance of operations like "hg diff" and "hg status", particularly where your repository contains a lot of files.

Why did I have no change in modification time? Well, my source file was generated by a script that I have written to convert SCCS change history to Mercurial commits. If my script can generate two revisions of a file within a second, and the files are the same size, then I run afoul of this shortcut.

Solution - I will just change my script to apply the modification time from the SCCS history to the file prior to commit. A "touch -t " will do that easily.