Set-up: multiple stacks attacking a city. The first stack loses.
Result: all the other stacks don't attack (don't move, even).
I've seen this before, but always thought some of my stacks must've been intercepted (I don't pay too much attention to turn review normally as I'm pressed for time often). But this time I was attacking a neutral withough any interference around. The first stack lost, all the other stacks didn't execute their move (and thus didn't attack).