Whether due to lack of prep (“Crap, there’s no way this’ll last four hours”), an encounter taking a lot less time than you expected or any other factor, have you ever padded out a gaming session to make it last longer?
Ways to pad a session include adding an encounter on the fly (“Ninjas kick in the door!”), making a puzzle more complicated than it should be or drawing out a conversation between the party and a key NPC.
Doing things like this to make the game more fun, or to adapt to changing conditions, is part of being a GM. Doing them just to stretch out a session is padding (the key difference is intent).
And as a follow-up question: Is there such a thing as good padding?