When a friend asks you how big your ex-girlfriends boobs were, but you say they we're one size larger.
Idiot 1: They we're Z cups Dude!
Idiot 2: O.... M.... G!
"Brandon always Overchestimates his ex-girlfriends; That guy is an idiot."
To guess a girls breastsize only to find later that the extra cup size came with the bra...
I totally overchestimated Alli... She looked to be a solid B, only to find out she was barely an A!