Not reading carefully enough
I'm sure most of us have mis-read something and thought "What the.....", then gone back and read it again to discover what was really written.
I surpassed myself today by reading "We are bringing a 9 year old catholic in July. Does anyone have experience for eating out" and naturally thought "What does being a catholic have to do with eating out?" Reread the question and the word was actually 'coeliac'. That'll teach me to be properly awake before starting to read.