One of my personal pet peeves would be the nonexistent word
'alot'.
I know it may seem like a small thing, but honestly, I get pretty
worked up every time I see it.
One day, while I was reading a certain book for research, I came
across this (circled):

Alexander, R.M: The History of Fish
Mechanics, in
Webb, Weihs (1983): Fish Biomechanics
Arrrgh.... it's 'a lot', not 'alot'!!!! 