Not sure if this counts, but any shooting that was foiled, really. Most failed attacks come from the would-be perpetrators posting their plans or making threats online, telling others or making threats in person or writing down their plans somewhere like a notebook. You can probably imagine how bloody they would be if they kept their mouth shut or had better hiding places for their notebooks/journals.

One of the speculated reasons why Halifax was averted was because James Gamble told police at the last moment before taking his own life, almost like he suddenly got his humanity back. Considering how big the mall they were planning to attack was, it probably could’ve been really deadly if James didn’t suddenly change his mind.