mtn: misuse: rename target 'Foo/bla' already exists

Can anyone tell me what I should do here:

mtn: updating along branch 'mybranch'
mtn: selected update target 4e34dedb6e8a9cb16f3a303b86c61fd384328e42
mtn: [left]  5a641ac778ae8acab3f1415febc4582bc9204810
mtn: [right] 4e34dedb6e8a9cb16f3a303b86c61fd384328e42
mtn: adding Foo/bla
mtn: misuse: rename target 'Foo/bla' already exists

I was trying to check out the two revisions and compare the directories  
and files but I can't check out the left side:

mtn: misuse: no match for selection '5a641ac778ae8aca'

Before I mess up everything and lose revisions maybe someone can tell me  
how I can proceed safely? I'm using monotone 0.40 (both on Windows XP and  
Cygwin).

Boris