I know that blasts require LoS in 4e and that bursts require line of effect between the target square and the creature using the power but one my players asked if a burst would hit diagonally around a hard corner. My first thought was a hard no, but then remembered reading somewhere about bursts "splashing" around corners decided I needed some clarification. What are the rules for burst attacks and corners?
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From my reading of the PHB, both require line of effect from their origin square, and neither require line of sight (p272):
You are probably remembering a rule from 3.5 that allowed spreads to do exactly what you describe. But 4e is a bit simpler; you only check whether there's line of effect from the origin of the spell. (I'm sure there are exceptions, but those will be explicitly called out.)