Never - knowingly, anyway - had a client's animal experience it, but I had one of my own that I'm certain died of anaphylaxis after receiving a polyvalent Clostridial bacterin 8-9 years ago. We used to work the calves & turn 'em right out the gate - this one ran over the hill, laid down and died. After that, I hold them in the barnlot for at least 20-30 minutes before I turn them out. Usually keep some epi on hand now, but have never had to use it.
cg8, that vet probably accidentally gave an intra-carotid injection. They always warned us about them when we were in school.. My wife (also a DVM) gave one one time - she stuck the horse, drew back... blood looked too bright to her... she looked to me for approval... I said, sure, go ahead! Pony flipped over backwards. Luckily it was our own pony and not a client animal... but i had to sit on her (the pony) head for a while until the convulsions stopped. Guess THAT taught her not to trust me...