I develop *a lot* of Motion Templates and I push the envelope very hard a lot of the time. There are many times I have crashed Final Cut due to some of those "experimental" templates...
My Final Cut projects really aren't single purpose video developments. They are simply testing grounds for template development. They can get to be quite large and all the clips are "heavily laden" with effects, titles, etc... I have been known to add up to 20 Effects to a single clip at a time using Color and multiple Shape Masks for each added effect.
I am fairly sure nobody else uses Final Cut quite like I do.
I think I remember coming across the option to Verify & Repair in the past (somewhere around the time I learned about Quit and Keep Windows)... I just had never used it. I know how to recover from the crashes I cause and never really thought much of having to run a repair on the project... they always seem to run fine after I undo the problems I caused.
... But I tried Verify & Repair out anyway and was surprised to see this:

(this is what it looks like in 10.4.10)
After running Verify & Repair, there is no feedback as to the outcome, so running it again a second time resulted in:

Okay - so it fixed it — whatever it was. I don't know where the problems were and I don't know what caused them — useful information, I'd think.
I also don't know what would show up if the repair fails (and I hope to never find out!) but chances are, if the project is in that bad a shape, you won't get the option to run Verify & Repair.