New compression shut-off valve leaks
 

I just put on a new compression-fit shut-off valve and it's leaking. Is it possible I crimped the copper pipe when I took the old one off? If so, how do I fix this problem?

Hey Cheesefood:
Compression fittings are hard to mess up. Take an adjustable wrench and tighten the packing nut (around the valve stem ). If that doesn't stop the leak you can hold the valve with one wrench and turn the compression nut another half turn.
If that doesn't work, its time to take it off, cut the copper and start over with a new ferrule.
Glenn

Exactly what I would do.

I always use a little pipe dope around the compression sleeve. Even though you shouldn't have to, it prevent any miniscule leaks. Also, some to lubricate the threads helps get it tighter.

If the leak is from around the stem, just tighten the stem nut. That's very common.