When you see a picture of anyone looking surprised or horrified, you instantly say that they have just seen Gandalf the grey uncloaked.
I've had some very strange looks.
Or
At school / collage, you attach small wheels to a paper mashie volcano someone in another class has made and label it 'Mount Zoom'. Yes, I did do it!
Or indeed:
Whenever asked to write your occupation, you nearly write 'Downer'.

Not guilty... yet.