Anyway, you certainly have discovered some sort of bug. The log suggest this has something to do with the sources or changes file - does that ring a bell with you?
this reminds me (a little) the "bug" I had because of my anti-virus (NOD32) which was scanning the changes files each time it was accesses... Not sure this is it but as in linux you probably don't have any antivirus ;)
my 2 cents...
Cédrick