On an especially hot day in Northern California, I put down 2 liters of bottled water before climbing into a C172 for a trip to visit a friend of mine about 300 miles away. Now, this 172 had a bit of a rigging issue which was known around the club - one needed a goodly amount of left pedal at all times to keep the old girl from trying to swap ends in flight, which was a bit tiresome over the long haul but certainly manageable.
Manageable, that is, until the water ran through my system and demanded release. So here I am at 4,000 feet with a half-liter narrow-neck water bottle in one hand, the yoke in the other, and I can't remove my left foot from the rudder pedal. As if this wasn't bad enough, my long legs demanded that I reach down to trip the seat release to gain enough room for the gymnastics required to put Tab A into Slot B, as it were.
The 172 seats, as you may know, slide waaaaaay back into the passenger compartment, on short notice, which makes keeping a foot on the rudder pedals sort of impossible.
The resultant near-roll and mess in the cockpit taught me a very valuable lesson about making a trip to the head before departure. I shudder to think what the accident investigation board would have found after picking through the wreckage...