If You, As A Jungler, Screw Up Someone's Lane
By making a bad play, forcing your laner to follow up to try and prevent your death, then ending in a double kill for the enemy -- DO NOT leave your laner hanging especially if it's a delicate lane that is lost by one kill. Fix the bleeding before it's too late. Too many times I see jglers come in when the laner is pinging not to only to screw up any methodical planning the laner had causing him to be royally screwed for the rest of the game.