Clock running out

[Triad zebra]

This didn't happen in my game, but what if?:
4 tics remaining tied game
A1 throws it into A2 who beats the bad defense and is heading for a layup 50 feet away.
I notice clock hasn't started, I begin a count in my head and blow it dead prior to the layup. Just prior to blowing it dead, I notice the clock is now running and has 3 seconds remaining (obviously started 2 seconds or more late) I blow whistle, clock stops and there is 2.4 remaining. We're going to OT.
I'm sure there would be some angry people there, but is this what you would do?