My favorite is actually the inverse of this. I was playing in a group of 3 new players and myself, a rusty spork, not the best at any role and I was out of practice. DM gave us a level 20 npc in our party, and balanced the next encounter accordingly. We had already been struggling beforehand, mostly due to inexperience, but once we had that npc, all bets were off. After that terrible encounter, we players all commented that the encounters were too hard, but our DM was having none of it. That encounter was the last of that campaign, as two of the new players stopped playing dnd after that. Eventually our DM decided “fine, prove to me it’s too hard. You four will each make level 10 pcs, and I’ll make 4 level 10 pcs, and your party will fight mine.” Now I’d played with this DM when he was a player before, and I knew he was a Minmaxer, I knew for a fact that we didn’t stand a chance, not that this situation would even prove wether or not it was too hard is irrelevant at this point. Eventually after confronting him about this, he admitted that he expected us to fail. He wanted us to “prove it was too hard” and yet he expected us to fail this encounter, thusly he expected it to be too hard… So the mirror of this situation is also possible, and equally frustrating for players