When you are so excited or so angry that you lift your hand off the shift key to early and accidentally put a "1" at the end of your numerous exclamation marks.
Mother: GET HOME NOW !!!!!!!!!!!!!!!!!!!11
Child: Ugh okay!
Common typo among people who like to insert many exclamation points to the end of a sentence, caused by accidentally releasing the Shift key before the 1 key, and sending the message without previewing it to realize the typo had been made.
Often used on purpose to make fun of such accidents and the people who make them, or as part of leetspeak.
A similar but less-joked-about occurrence is the tendency to insert backslashes after a string of question marks.
A term originally created by a sergeant with the Los Angeles Police Department SWAT Team. Designed as a reminder that in fact you only have "one ass to risk"(1 as-te-risk), so you'd better be careful.