Does force quitting apps before they fully launch cause any damage? (OSX)

No… I have used OS X from the horrible days of 10.0 and have never had any problem doing what you describe. And I do it a few times every day.

Edit :: And I should add that I spend tons of time on Macintosh message boards and nobody has had a problem from doing this.

Depening on what the app is actually doing when you kill it, you may cause damage to open files like plists or prefs files. However, since most of what an app is doing with the file system when you’re launching are reads, not writes, you’re usually safe. If the app is setting up temp files or something when it goes down, you may still have those out there and in an uncertain state, which may cause the app problems the next time it launches. Again, usually you get away with it. Its just somthing to watch for if an app starts acting flaky.

As chaosrob pointed out, you’ll only do damage if something’s in an unfinished state. So, I wouldn’t force quit an app the first time it runs, but after that, you’re probably fine. I, too, have been using OS X since the public beta and have never had a problem from doing this. Gotta love Unix!

Only unsaved work, which includes preference files. Very unlikely tho. Never heard of anything like this.