I feel like a pillock. How about you?

I went to three autoshops/garages today specifically asking for dot 3 brake fluid (the tank states 'USE DOT3 ONLY'), all to no avail.

I turned and left each one with a quick, 'Thanks, bye'. It was only when I went to the fourth when they casually mentioned; 'No, but we've got dot 4 which you can also use. Small or 1 litre?'

So today I have learnt that the only difference between the dots is the boiling temperature, it's perfectly OK to use dot 4 instead of dot 3 in an older vehicle, and some people have no desire to dig any deeper than the original question.

How have you embarrassed yourself in front a mechanic/someone in the know?