Typing or texting a message to someone, without realizing that the caps lock is on, and sending the message to the person in all CAPITALS
Also typing in all capitals to emphasize or "yell" over telecommunications.
"HEY WHATS GOING ON?"
"whoops, sorry, cap attack"
"WELL YOU KNOW WHAT YOU'RE JUST JEALOUS! :@"
'Kay relax. No need to cap attack me.'