Here are some examples that I have experienced. Please share other examples also.

1)On a long hike and cell phone about dead.
2)On a long hike and GPS about dead, and no extra batteries
3)A long way from anywhere and no flashlight and almost dark
4)In an area with a lot of micros, and no pen-and no store around
5)get dropped off somewhere by boat and forget where you told someone to pick you up at(phone about dead and possibly no signal)
6)and my favorite- go a long ways to geocache only to find you didnt bring ANYTHING to cache with.

7) not enough drinking water!

arriving at a puzzle cache then realizing that the coordinates are the fake ones and you don't have the real ones with you (GSAK has a handy function to correct coords, BTW).
9) agreeing to go caching with KY3 then realizing that his tag line is "Drive Like A Geocacher!!!!!!!!" (kidding, dude)

Been there, done that

10) While out hiding a new cache in a wooded area, you lose your wife's cell phone

pcs, I have a solution for finding yourself w/o a pen.

B&G's Perspective

SBCP's Perspective