At the LGS I saw one of Gary's examples in the flesh.

A guy came into the shop with an AR-15 receiver he said he was building. He was having a problem. I didn't hear everything he said but part of his problem was the disconnector was not disconnecting.

They looked at it a bit and played with it. Seems either the trigger group or the receiver was not made/finished/whatever correctly. Things were out of alignment.

The gun shop did not seem overly anxious to mess with it.