This thread has reminded me of a rather embarrassing incident that happened with one of my kittens many years ago - embarrassing to me, not the kitten.
We hadn't had our two kittens long when early one evening one of them did a poo in the litter tray that had a whole lot of bright red bits in it.
I thought it was blood and in a panic rang my vet who told me to bring the kitten in straight away.
I duly packed up poo, kitten and children and rushed off to the vet.
Vet looked at the poo and said 'it certainly looks like blood'. However, after he'd had a good dig through it he realised pretty quickly that it was bits of plastic.
My children were really keen on those plastic model straws that were popular in the 80s, don't know if they're still around, but kitten had clearly been playing with one and had chewed her way through it.
Vet and I had a good laugh about it and, bless him, he didn't charge me for my outside hours emergency appointment. But, boy, did I feel stupid.