I can quickly recall three events where finger on the trigger resulted in NDs:
1. Experienced officer and his rookie are covering a meth freak/forgery suspect hiding in a dog house after a car and foot chase. Experienced officer has his finger on the trigger of his P-35 and during the excitement discharges a round which narrowly misses the rookie's leg and impacts the ground between the rookie and suspect (who quickly surrendered).
2. Experienced officer is involved in car chase with stolen car and upon the stolen ride crashing out, jumps out with finger on trigger of his P-35 and begins foot pursuit. Officer NDs a round and being startled by the ND, NDs a SECOND round, both of which hit the ground.
3. Multi agency search warrant against armed meth dealer goes awry when a very violent fight with the suspect occurs at front porch/door of the target residence. Involved in this fight are myself and three other officers and the puke du jour. During the fight, an experienced (NRA Master PPC shooter) shooter and officer with his finger on the trigger of his S&W 686 4" loses his balance and falls backward, launching one round of WW .357 Magnum 145 STHP. This round tracks between my throat and right shoulder (very narrowly missing) and strikes the officer's partner, critically wounding him.
Yours truly, having heard the shot and the accompanying scream and blood splash (arterial wound), deduce that the meth dealer has shot the police. I draw my 1911, clear the safety and slack out the trigger to deliver eight rounds amidships to the puke (which were going to be fired one handed at a range of about four feet). Just as I'm about to break the first shot, I see the dealer's hands are empty and also see the revolver in the hand of the officer in my periphral vision to my left side. By God's grace (and that's all I can say stopped me) I kept from shooting this guy. My decision to shoot and then not to shoot couldn't have taken 3/4 of a second in the dark on that porch. Like many LE deals where one mistake is made this deal snowballed into a beating of the suspect, a big IA deal and an FBI Civil Rights investigation. All for one finger in the wrong place at the wrong time...