I was checking in online last night for a flight and came across a classic example of asking the user for perfect user input when it would be easy for the programmers to simply clean up any bad data.  In situtations like this I believe the onus is on us, developers and programmers, to make our users lifes easier not ours. 

Lets look at an example, the Southwest online check in screen which has the following bizarre instruction: "Confirmation Numbers use the letters O and I instead of the numbers 0 (zero) and 1 (one)."

Fixing any user input on the server side could be achieved very easily with a simple RegEx making the page both easier to use and clear of instructions.

A quick note about Southwest: I actually have a lot of luv for Southwest and without them would see my relatives a lot less.

Another quick note: I'm sure I have made my users lives harder than need be in the past with fields like this (though I have also discovered a recent fondness for the masking features in cfform).