If I recall correctly, it was also the end of a very long day of shooting (in addition to the illness). That "I don't have time for this" look on Ford's face is real. He wanted to finish up for the day and probably get right to sleep.

Since we are talking Indiana Jones and jimbo's almost apparent lack of BTS knowledge...

You do know how he kept the fedora on, right?
You do know Ford used a staple-gun on his head to secure that hat. If you don't believe me, I can get screen-grabs from the DVD set. They used footage of it in the menu system.

Also...
*slaps jimbo with a medium-sized fish, as mentioned earlier.