Yeah I agree. But the problem is I can't even figure out what the gag was MEANT to be.
What we know is this:
- Indy ripped
something
- Mutt thought the result of that was bad
That something was not his pants clearly. So it must have been something else, of no consequence - the tarp or something.
If Mutt knew it was of no consequence he would have said nothing.
Therefore Mutt thought something had gone wrong. But what are the possibilites?:
a) he thought Indy ripped his pants
b) he thought he ripped the tarp and dropped and knife
b is not likely as we have no reason, as the audience, to think that sound indicates a dropped knife. We DO however immediately think "uh-oh he ripped his pants" making it the most likely explanation.
But then why does it seem so strange? Because of Indy's reaction. You'd expect him to smile, or say something like "na that wasn't my pants" or something to that effect. But he doesn't.
He frowns and gets on with it, which I think is where the audience gets lost. He acts pretty much the same as he would if his pants had really been cut. So without anything to negate the ripping sound, we as the audience, get confused.
I think that's the best over the top, got-nothing-better-to-do-with-his-time-than-think-about-harrison-fords-pants analysis I can give.