This is not poking fun at someone making a mistake, we all make them. This is a hilarious video of a guitar player showing they are in fact human and messing up an intro to a song he’s likely played over 1000 times.
The fact Kirk Hammett acknowledges the mistake makes the video that much better. He melts into the floor knowing he made a mistake, and then 40,000 plus fans laugh and cheer Kirk Hammett, and the show goes on, as it should.
This is likely more common place than we know about, but maybe you’ve been at a concert and slightly under the influence so you don’t notice whatsoever. I’ve been to dozens of concerts and can’t say I can vividly remember any musician screwing up, but I’m not a pro and if they repeated a verse, or sang a chorus again I would just blindly think that’s how they play it live, and if it’s a good song who cares.