FFXI: Sneak + Invisible / Sac pulling
WOW: Stealth + Lockpicking chests (And in some cases soloing bosses). You could also do what people are doing in these dungeons to skip to bosses, albeit its probably less efficient in most cases.
UO: Invis, Hide, Stealth. Mark/Recall onto boss spawns in dungeons.
Lineage 1: Ignore monsters and run straight to the boss (Or use a cloak of invis/invis spell/Absolute barrier)

This is possible in many MMORPGs in many dungeons. While it - MAY - be an oversight, its a far cry from an exploit.

And on that note I've shown how old I am.. lol..