diff --git a/mercurial/merge.py b/mercurial/merge.py
--- a/mercurial/merge.py
+++ b/mercurial/merge.py
@@ -1404,6 +1404,31 @@
                 atomictemp=atomictemp,
             )
             if wantfiledata:
+                # XXX note that there is a race windows between the time we
+                # write the clean data into the file and we stats it. So a
+                # writting process meddling with the file content right as we
+                # writes it could cause bad data to be gathered.
+                #
+                # They are 2 data we gather here
+                # - the mode:
+                #       That we actually just wrote, we should not need to read
+                #       it from disk, (except not all mode might have survived
+                #       the disk round-trip, which is another issue)
+                # - the mtime,
+                #       That could be accurate enough on filesystem with
+                #       nanosecond precision, but is read in a racy way.
+                #
+                # (note: we get the size from the data we write, which is sane)
+                #
+                # So in theory the data returned here are fully racy, but in
+                # practice "it works mostly fine".
+                #
+                # Do not be surprised if you end up reading this while
+                # looking for the source of some buggy status. Feel free to
+                # improve this in the future, but we cannot simply stop
+                # gathering information otherwise `hg status` call made after a
+                # large `hg update` runs would have to redo a similar amount of
+                # work to restore and compare all files content.
                 s = wfctx.lstat()
                 mode = s.st_mode
                 mtime = timestamp.mtime_of(s)