Post pics or it didn't happen
Please do some research before posting non-helpful suggestions-they waste space and help no one.
As for the original post-the unwanted screen capture phenomenon seems to appear on other devices. I have noticed similar posts regarding your problem and it seems to be a software issue and was happening frequently and under a wide variety of circumstances-texting people, reading emails, watching movies, etc.
After some poking around, I have found a possible solution posted by another user that you may want to try. This involves using the commands in the phone dialer to update the key software.
Steps:
1-Open phone dialer
2-Press the buttons
INSIDE the parenthesis (*#*#2663#*#*)
3-Look at 2 different touch screen firmware versions.
4-I am too lazy to rewrite the screen here, but you dont need that info. Just look at the version number. If they match then you have another problem. If they do not match 22/21 etc, then update them one at a time and your problem should be fixed.
Goodluck.
*I found this method to work on my Samsung Vibrant. Hopefully it will work on your device.