I heard of a case where a lost dog was adopted and rechipped.. and after the fact a chip was found in it's leg.
The dog was eventually reunited with the original owners, but it was even more heartbreaking because they'd had the dog for weeks.
Or another case where a dog was chipped and then the vet couldn't find it on a later examination.. it had slipped into the chest or something weird..
But when I bring stuff liek this up, I get told that it's rare. I'd just be worried that one of the rare cases could be my own dog..