In a game (very long time ago) where I was a player, the GM gave a map of a town for us as normal. It was the original map that came with the scenario or campaign. We ran after some guy who suddenly disappeared, so we started looking at the map for where he might have gone. Around where he disappeared there was an "X" on the map, with text saying something like "secret door to underground tunnel". We asked the GM, because we thought we were looking at a player map, and assumed that had to be some kind of common knowledge. The way he blushed revealed it wasn't a player map.
The GM should never be lazy with his maps, I guess. Always
make a separate map for the players.
I wonder if there should be a separate thread for bad designs in commercial products? Probably not...
D&D scenarios are pretty notorious, I think, in the way they have irrational and ridiculous combinations of monsters around in some ruins or dungeons. In Caldwell's Castle (? or something like that) there were kobolds near the entrance, wolves and bandits in the back (where they had no alternative entrances). So, how did situation come to be?
But even worse, I once saw (can't remember where, unfortunately) in a scenario for some other game, a dungeon, where some dragon (IIRC, there was very little point in there being a dragon there, but there it was) living in a room in the middle of the dungeon. The most ridiculous thing was that the dragon was big enough to nearly fill the room. And yet he had no way of moving in and out of the room, as the rest of the dungeon was too small for that, and there was no roof exit either!
So badly designed adventures, if not exactly GM blunders, give opportunities to newbie, unthinking GMs to make blunders, by actually using the adventures, or at least by using them as written. I'm not sure how much could be salvaged from such trash, though, even by a thinking, mature, experienced, good GM.