One I am noticing a lot lately is 'threw' when it should be 'through'.
Wow I had a test in this today, and I thought about your post, that's awesome!
Words like Baby people are fine with, when you have to make babies, quite often I see parents doing something wrong and ending up with disabled babyies.
Plenty of words similar to this that have same rule, drop the y before adding ies.
That was the first that came to mind.