What's your biggest bike related mistake?

[PaulSecteur]

In 1986 i ourgrew my raliegh striker.

 

[Theseus]

Getting a bike with an ISIS bottom bracket.

Wore out in weeks and a pain to get out as the splines are shallow. ... now replaced with a proper square taper.

 

[mjr]

But I neglected to swap saddles, and I gave away my lovely old worn-in Brooks B17!

Amen! Forgetting to swap saddles when I donated my old bike to charity is probably second-placed to my tyre-cleaning debacle.