Though this brings us to another "unfun" thing:
When the DM expects you to run away, but also tries to enforce the rules to prevent you from running away.
Most editions of D&D have a problem where it's not very easy to run away, because of the way movement and rounds work. 5E is very much one of them - it's virtually impossible to escape an enemy who can move at the same speed as you, and completely impossible to escape an even slightly faster one (barring magic or similar).
So sometimes a DM sics an insurmountable foe on you.
The correct reaction, and the one the DM wants, is to get away from it. But unless the thing is literally slower than you (which, say, 100 goblins are not lol), that requires the DM's cooperation, and it really requires the DM to not want to play out chasing your or whatever (rather roleplay it out or the like). The trouble is a lot of DMs don't get this. They have unreflected/unexamined belief that you can run away, and they try and play it out. And then you can't. Indeed if it's something like a dragon, it just catches you. I remember once making us run from a dragon, then having it chase us, and you could see he was genuinely surprised when it easily caught up to us. It's like buddy, did you not do the math?
So if you want the PCs to flee, let 'em flee. Even drawing it out a bit is unlikely to be as fun as RPing it without the rules.
Sometimes I've just narrated it but I prefer to do it as a challenge with group and individual checks. But I agree, if you want the group to run, give them clear exit signs.
Also give some thought to what happens if they fail and what the consequences could be.