The mysterious stranger is just a grumpy old drunk. "Quests? What yer talken about? Gerraway you annoying young whippersnapper".
Subverting the trope merely to show the players they had made assumptions is not all that constructive, though. It feels like a "gotcha!" moment.
Subverting the trope to support a theme, or an interesting difference in overall plotline, however, can be really good stuff.