I remember late one rainy night, after 7 days of instructing many hours a day, and then doing a 13-hour, seven-leg trip for a Part 91 operator, I called clearance delivery at OAK. It was for the seventh and final leg of the day/night, and my call probably sounded something like this:

"Clearance, Cessn...I mean Senec...Beech...darn it...Jetprop 12345, IFR to...to...let's see here...you know what, I'm just going to shut down and call you back, 'cause I can't even remember where we're going."

That was the night I realized what duty time limits are for.

(It all ended well.)