When you suddenly realize the meaning or derivation of words you've been saying all along without questioning them. Sometimes it can be shocking, sometimes it's just cool.
The only one i can think of now is when i realized that the opposite of inept is adept (i'd been trying to invent the word ept and realized i didn't need it).
(later) Oh, wait, here's another: I became briefly enchanted with the word indeed when i split it into its component parts.