This evening I've been reading a biology book and I thought I actually caught a typo so I looked it up. The author was talking about reproduction and used the phrase "sperms and eggs." I took biology in high school and at the university level and never realized the plural of "sperm" is "sperms." I would have bet a small amount of money that its singular and plural forms were both just "sperm," sort of like sheep and sheep.
I'm guessing that both are probably acceptable. After I guessed that I checked a few other dictionaries and some actually list both as plural. The author of the book is British, so that could explain why I haven't seen that particular plural form before.