never heared of this again, please reopen if there still is a problem.

Jeremy, as I couldn't reproduce the problem neither before nor after the change,
can you verify if it is fixed?

The revert changes just committed to tip might fix this, please test.

When merging two heads, and then using 'hg revert' to cancel the changes, 'hg
diff ' gets an exception for removed files.  'hg diff' for normal removed files
works as expected (shows a diff of all lines removed).

For example:

: abulafia:pts/7; hg heads
changeset:   49:9e9788fc1a7f
tag:         tip
user:        jeremy@localhost
date:        Tue Oct 25 14:44:39 2005 -0700
summary:     Added tag wanted for changeset 10583d8e7f254fcaf9cdf6a1834268c4c126c2a8

changeset:   47:8c88cdabb6b0
parent:      22:350932131763
user:        jeremy@localhost
date:        Tue Oct 25 14:30:32 2005 -0700
summary:     add bonk

changeset:   46:a49e8b2d2200
parent:      33:247199aaec38
user:        jeremy@localhost
date:        Tue Oct 25 14:30:04 2005 -0700
summary:     add biff

changeset:   45:20d004d483b5
user:        jeremy@localhost
date:        Tue Oct 25 14:29:38 2005 -0700
summary:     add bar

: abulafia:pts/7; hg update -C 49
: abulafia:pts/7; l
total 24
4 drwxrwxr-x   3 jeremy jeremy 4096 Oct 25 14:53 .
4 drwxrwxr-x  15 jeremy jeremy 4096 Oct 25 14:52 ..
4 drwxrwxr-x   3 jeremy jeremy 4096 Oct 25 14:51 .hg
4 -rw-rw-r--   1 jeremy jeremy   48 Oct 25 14:51 .hgtags
4 -rw-rw-r--   1 jeremy jeremy   26 Oct 25 14:55 foo
4 -rw-rw-r--   1 jeremy jeremy    7 Oct 25 14:51 splock
: abulafia:pts/7; hg status
: abulafia:pts/7; HGMERGE=false hg update -m 47
merging foo
merging foo failed!
: abulafia:pts/7; hg status
M bonk
M foo
: abulafia:pts/7; hg revert
: abulafia:pts/7; hg status
M foo
R bonk
: abulafia:pts/7; hg diff
** unknown exception encountered, details follow
** report bug details to mercurial@selenic.com
Traceback (most recent call last):
  File "/usr/bin/hg", line 13, in ?
    commands.run()
  File "/usr/lib/python2.4/site-packages/mercurial/commands.py", line 2412, in run
    sys.exit(dispatch(sys.argv[1:]))
  File "/usr/lib/python2.4/site-packages/mercurial/commands.py", line 2567, in
dispatch
    return d()
  File "/usr/lib/python2.4/site-packages/mercurial/commands.py", line 2552, in
<lambda>
    d = lambda: func(u, repo, *args, **cmdoptions)
  File "/usr/lib/python2.4/site-packages/mercurial/commands.py", line 1048, in diff
    text=opts['text'])
  File "/usr/lib/python2.4/site-packages/mercurial/commands.py", line 303, in dodiff
    to = repo.file(f).read(mmap[f])
KeyError: 'bonk'
exit status 1
: abulafia:pts/7;