Okay, first I'm not a grammar Nazi. I really don't care if anyone around me butchers the English language. However, being the "cheerleader" that I am, I use the word "Yay!" a lot. Even here my spell check is red lining that word.
For years I used "Yea!" instead of "Yay!" which is incorrect. I found a helpful site to help in the rare case that I'm confused on which word to use.
http://public.wsu.edu/~brians/errors/errors.html#errors
As for Yea/Yeah/Yay:
“Yea” is a very old-fashioned formal way of saying “yes,” used mainly in voting. It’s the opposite of—and rhymes with—“nay.” When you want to write the common casual version of “yes,” the correct spelling is “yeah” (sounds like “yeh” ). When the third grade teacher announced a class trip to the zoo, we all yelled “yay!” (the opposite of “boo”!). That was back when I was only yay big."
So, what other words do find are often misused?