It was the first time I've ever seen it.
I'm trying to think of other conditions that were specific to this instance. One thing I can think of is both 25pdrs that fired and been undetected up to this point. Another thing is both tanks fired upon had been stationary. The only reason the 25pdrs didn't fire earlier is smoke had blocked visibility. Once the smoke cleared, the guns fired.
I know it will be difficult to fix if you can't recreate the problem. One of the first things I do at work is try to recreate problems when users tell me some program or process isn't working correctly. It's a bear to fix if I can't do that.
It's probably not too unrealistic for a gun to fire a shot or two into the smoke if the target had been stationary when it started firing. If you figure the shots had been close and the target might start moving, a shot might connect if the target moves into the path of the shot.