Ed will become the opposite of his Gentle Giant self if he has a pebble in his shoe, which is kind of like an animal going berserk if it has a thorn in its paw. And it would be foolish to mess with his sister or his pals.
In the episode "Little Ed Blue," Ed gets into a bad mood that drives him to stand up to his bullying baby sister Sarah ("Ed! You can't kick me out, I live here, too!" "So move!") and threaten his friends Edd and Eddy ("Touch me again, and I will squash you!"). Edd is incredulous when he finds out it was caused by a small rock in Ed's shoe.
Don't try to interrupt him once he's started watching an eight-hour monster movie marathon.
In Ed, Edd n Eddy's Big Picture Show, hurting his friends is revealed to be one of Ed's when Eddy's Brother got a kick out of brutally beating Eddy and Edd in one of the few instances where Amusing Injuries weren't in effect. Ed responded by unpinning the door he was pulling Eddy away from, sending it slamming into his face and knocking him out. This was the only time in the series that Ed consciously intended to hurt someone.
Edd is a Neat Freak, so sneezing on him would be one of the worst things you could do to him. He also gets angry when Ed and Eddy play pranks on him (As seen in My Fair Ed and A Case of Ed). But, in Ed, Edd n Eddy's Big Picture Show, him falling victim to a cruel prank finally sets him off.
Do not call Eddy short. Do not take money from him. And do not ruin his scams.
Ed and Eddy's mere existence seems to be another one for her.
Similarly, don't trash her room.
In a few episodes, Jimmy is shown to get angry when some of his toys are destroyed. He also doesn't take kindly to being pushed around, but when he was bullied and given a wedgie in "If It Smells Like An Ed," it was the straw that broke the Camel's back.
Also, do not take Sarah away from him, unintentionally or not. He foamed at the mouth and ripped the head off of his stuffed bear.
Only the Eds have been known to push Kevin's buttons, whatever they are.
Don't steal or destroy Kevin's bike. It won't end well for you. (Unless you're Nazz.)
If you take Plank away from Jonny, it can get really ugly. To wit, he once took on all three of the Kanker sisters to get Plank back, and he won.
For that matter, don't threaten Jonny around Plank, and DEFINITELY don't harm his parents. He knows people.
Eds, upon seeing Plank flying towards them after stunning Jonny with a camera: "PLANK'S GONE MAD!"
Speaking of Plank, it is suggested at one point that he doesn't like board jokes.
Nazz will get preachy and defensive if you've done something wrong.
Don't mess with her hair, either. Seriously, don't.
Rolf takes pride in his customs. He does not take kindly to those who ridicule the customs of his homeland. Do not tarnish them, or he will beat you. With a Fish.
Rolf: "Son of a gun, you insult Rolf by denying the eels?!"
Also, wolves. Mentioning wolves around him seems to flip a switch in his head.
The Kankers have 2 of them: the first one is if their "boyfriends", the Eds, are paired with someone else outside of the girls' intentions. The other? Don't take their Ship-Inna-Bottle. The resulting rampage was treated like a natural disaster.