I have also had the same issue just now. This was really quite freaky -- tought I had gone senile or something, couldn't remember a password which I use essentially every day. Fortunately I could just go back a few versions in dropbox and recover a working kdb-file. Now I just wonder exactly how many logins I have lost...
Like others above I am also using dropbox to synchronize; using regular keepass on my windows box and keepassx on my office ubuntu desktop.
Something probably goes wrong when dropbox tries to synchronize changes -- does anyone knows if dropbox tries to do some kind of merging of two files that were both changed since last sync? I hope not -- If that was the case, then it could obvious screw up a lot of things, but then this would be an issue for a very great deal of files/applications with some kind of encryption.
I wonder if this also can happen if you are only using regular keepass? Or if it only happens if you are using the windows-keepass + linux-keepassx combo?
Up until now I really did appreciate this combo which was the best way I have yet come across to have my passwords when I need them.