While working on a software package to move Odd Lot orders/reports between brokers and the Odd Lot dealer, we were doing our usual Q/A, including [what we thought were] every possible bad input from brokers and Odd Lot dealer.
Aside from the usual OS-MVT consoles, the system was controlled by operators manning 83B3 teletypes. One of these operators said she could crash the system any time. When challenged, she planted her elbow firmly on the keyboard of an 83B3, sending utter garbage into the system. She did indeed crash the system and we realized that garbage input could come from unexpected places.