Yes - not always intentional "bad manners".
When flying domestically in the US, boarding passes are checked at the gate, but not again at the door of the plane. For international flights, the passes are checked again at the door, which a frequent domestic traveler may forget, having put the pass away while walking down the jetbridge.
An example - a couple of years ago I flew AA LAX-JFK-LHR - we were late arriving JFK, requiring a sprint across JFK from one terminal to another (not very familiar with that airport, either), out of security and then back in. As one of the last pax to make it to the LHR-bound flight, I arrived at the gate breathing heavily having run the last stretch from security with untied shoelaces. I presented the boarding pass to the staff at the gate, then put the boarding pass away as I made my way down the jetbridge, attempting to reorganize my shoelaces and trying to make sure I had not left anything at TSA during my rushed security check.
Upon arrival at the door of the plane, I stepped inside, still attempting to catch my breath, to be met by a FA who demanded my boarding pass. When I looked at her quizzically, unable to get any words out, she asked, in the manner of someone who was about to call security - "do we have a problem, sir?".
In my opinion - totally out of line, mean spirited and lacking any empathy to someone who was obviously extremely rushed to make the connection. I don't think I've ever been made to feel less welcome boarding a plane.