On either my first or second game, before i'd developed the habit of checking the fire selector on my gun (a hired m4), i decided to execute an awesome one-man-super-dooper-outflanking maneuver, and as the enemy pushed up the road with the vehicle, spent about 10/15 minutes making my way behind them.
I'd just managed to get into the building they were regrouping outside of when i was spotted, so trained my sights on the door they'd be entering to get the drop on them.
One creeps in, i pop out and get hit to the sound of "AWW FOR F*CKS SAKE...f*cking SAFETY!!".
Needless to say, i spent the next few games constantly checking the selector every 12 seconds.