I was shopping at Costco today and saw a sign, above a pile of clothing, that read MEN’S PANT. They also sell WOMEN’S PANT.
This misuse has been around for a long time (more than a century, according to this site), but I’ll never get used to seeing it. It bugs me every time.
Some people can’t understand that some words are only plural.
Another such common mistake is bicep. People seem to think that you have one bicep on each arm and hence two biceps in total. No, you have one biceps on each arm; the name is plural because of the double attachment to the bone. At the back of each arm, you have one triceps, which has three attachments. On each thigh you have one quadriceps. So many attachments! So many opportunities for detachment! I tore one of those attachments mostly off in my right biceps years ago, but assume that muscle is still my biceps, not my unicep.
I haven’t yet heard anyone refer to cutting paper with a scissor, but I’m expecting it to happen.
This is another common and somewhat related error. I just ran into it on Facebook: An individual being referred to as a homo sapien. I suppose people think that sapiens is a plural form, whereas it’s simply a Latin ending, and singular.