Brilliant thread!
Not quite us, but when we were on Harris and Lewis for one of Simply Paul's do's a couple of the other cachers attending found an unexploded bomb and had the Bomb Disposal crew out to "remove" it!  They really enjoyed that.
On the same trip we were driving down through Lewis when we were flagged down by a woman who's house had smoke just starting to come out of the roof, the roof space was on fire, that had been converted into a living area and her Sister was till up there.  While L rang 999 from her mobile, there was no house phone, I tried to get the sister out but, sadly, failed.  Had lots of forms to fill in at the Police Station afterwards - an experience we don't wish to have again.
On a lighter side, there's a bit of an old toilet cistern near LQ:Worcestershire, somebody found that and reported the cache as trashed!
We've found some monster slugs (and inadvertently picked a few up - ugg!) with caches that have been in holes and in trees.
Looking forward to hearing more stories....