This happens with some regularity with Horizon flights between BOI and PDX. At both airports, there are multiple outside gates accessible from the same exit from the terminal. there are always signs indicating each plane's flight number and destination, but people still manage to get on the wrong plane. Not too long ago, a woman was sitting in my seat. She snarkily informed me that I was wrong about her being in my seat. At least until she was asked if she was flying to BOI. She muttered something about San Jose and scurried off the plane.....