In the rein in free rein spelled like this (REIGN) or like this (REIN)? Technically, it's R-E-I-N. As in, a horse's reins are held so loosely that it's free to do whatever it wants. But the other version seems logical, too. If you're reigning over a kingdom, you're probably free also free to do whatever you want.

That's an eggcorn, a phrasing mistake that makes just as much sense as the correct expression. Linguist Geoff Pullum coined the term in 2003 as a nod to people's habit of mistaking the word acorn for eggcorn. Great name for an egg-shaped, kernel-y thing, if you ask me. 

Having a bad rap -- rap in the punishment sense -- means people talk negatively about you. But it's often mistaken for bad rep, short for reputation.

And did you think it was coming down the pipe instead of coming down the pike? You're not alone. Let us know what other eggcorns trip you up in the comments.