Bill Zebub
“It’s probably Matt Mercer’s fault.”
Right. But by definition hard task (DC20) is within the ability of unskilled schmuck (relevant bonus +0.) So it means any commoner would autosucceed at any hard task without catastrophic fail conditions and when under no time pressure. And this doesn't seem right to me.
I wouldn't get stuck on the terminology that WotC uses when giving advice on setting DCs. Your definition of 'hard' might mean a DC of 21, which avoid the problem you describe.
But, ignoring that, let's say we have a task with a stiff DC of 20.
First, fail conditions don't have to be catastrophic. (Not sure how you chose that word.) Just meaningful. It could simply be that you ruin your thieves' tools. So you could tell a player, "It looks like it's beyond your abilities, but maybe you could do it. However, if you fail you might ruin your lockpicks, which will require a second Dex check."
Or maybe you don't want there to be a consequence. In which case you can say, "It looks really, really hard, but if you try long enough you'll get it. It might take hours."
Maybe that doesn't sit right with you for whatever reason, but I don't see a problem with it.
And you didn't answer my question about knowledge skills. There often is no time pressure with those, so do you just let players to autosucceed in any with a DC that their skill+20 can beat?
Sorry, you're right. I personally see knowledge checks as something entirely different, and in fact I sorta wish knowledge skills were in a completely different category. I think I've seen @iserith say that they want players to describe how they try to recall some information....goal and approach and all...but I've never found that satisfying. I don't really see that kind of knowledge check as an attempt at anything; it's just asking if your character happens to know something, and it's randomly determined (or not, if the DM chooses.)
Yeah, sure, roll the dice and if you succeed the DM gives you the info, and if you fail the DM doesn't.