Contradiction???

It's okey to move a piece twice in the opening if there is a tactical justification, in this case there is, if you don't move the piece, you lose it.

These tips, do and don't for the openings are guidelines for beginners, and that's pretty much it. They shouldn't be followed blindly, but they are something to keep in the back of your head.

I love these kind of tactics.

Your opponent probably also "lost" a move because of which your piece was attacked. It seems you are referring to 1. e4 e5 2. Nf3 Nc6 3. Bb5 a6 4. Bxc6

Tricklev's right - all rules have their limits, you've got to understand them before you know when you can break them. once you've climbed the ladder you can kick it away (or perhaps better - you eventually learn to kick away your own scaffold and hang free...)