I am having a similar terrible fate like the original poster.
I just wrote the bug here:
Google decided that my pages .php should be handled like .php/
I haven't touched my site for half a year (working at other projects).
Nobody is touching the server. Nothing have been set to create "the redirect".
So, why is Google using the .php/ format instead?
Imaged doesn't load when used in that way. So users think that my site is broken, and the jump back. That causes me in 2 weeks to fall in rankings (and to lose the traffic).
Practically, Google bug killed half my traffic.