I was a little groggy last night but am remembering more this morning.
This may be what was described in my armorer class that was claimed (by S&W) to be a fault of a special FBI specified trigger - intended to improve the trigger pull - that could over-ride the draw bar, totally locking up the pistol.
With the trigger and draw bar unmovable, the hammer would not move either, so the slide could not be retracted. If loaded, the pistol could not be unloaded, but neither would it fire.
The trigger was spec'ed for FBI pistols, but made it's way into pistols meant for other police contracts and for the commercial market.
At least, that's the story S&W told....