Holding my hands up here, I really don’t.

The best I could do is examples where clearly the wrong person was cast and quickly replaced; take Geneviève Bujold as Janeway, or Eric Stoltz as Marty McFly.
I thought with both it just wasn't working out. That Bujold didn't really jibe with the grind of American TV and Stoltz and Zemekis parted ways because the tone was wrong. According to this, they shot for
four weeks, before deciding it wasn't working out. Stoltz was/is a great actor, but, wasn't right for doing a comedy. And they replaced him. That's hardly, a "meh, you'll do" from the director.
I think there is a tendency to cast based on fame, which is kinda “Meh they’ll do.” Bujold’s casting was like this I imagine, and though it’s not an opinion shared by everyone I would add Scott Bakula into the mix here. I think it was a case of “We can get Bakula therefore we must” irrespective of whether he was suited to the role (and he wasn’t remotely IMO)
I think more of a Paris Hilton in whatever horror film that she did as casting based on fame, and might be a good example of "meh, you'll do."
Was Bakula, though, their first and only choice? It's not like he's so famous that everyone else fades away. I could believe that argument if it was like, "Hey, we can totally get Vince Vaughn, he's a big Star Trek fan, let's get him." Which would've been a hilarious disaster.
Bakula didn't work out, but, I think the scripts were so shitty that I don't know who would've survived.
And talking of Enterprise, and I know he’s not a lead but he is a main cast member, I refuse to believe that Anthony Montgomery was the best actor they could get to play Mayweather in Enterprise? If ever there was an example of “Meh, he’ll do,” it’s there.
Yeah, this might be a good example. Maybe he had a killer audition, because he displayed the charisma of a console. Oy.