A sudden period of feeling generally down and/or depressed after attending a convention. It is usually preceeded by a time of being "high" on endorphins and energy, feeling really great about the con before it hits you like a ton of bricks.
Con drop can happen anywhere from a few hours to a few days after leaving, and is accompanied by an intense need to go to another convention or be with friends who were there as well.
Someone in con drop will be particularly susceptible to signing up for the next convention they're told about.
Dave: Hey, I just got an email about FoodCon 2011. What do you think about going?
Joe: Dude, I'm in con drop! Now I have to go sign up for it!