I looked at my gallery one time and I saw that I accidentally print screened an image. I was wondering if you guys know how I did that? My phone is a Galaxy Ace, and I have no app for that. It just happened.
On the Samsung Galaxy range of phones, running Samsung's TouchWiz enhancements to Android, you can take a screenshot of the screen by holding down the Back button and tapping Home.
This doesn't work in some apps that take an action immediately when you press Back, but as most apps don't carry out the action until release the Back button, this will work for them.
See also this previous question: taking a screenshot of an android phone?