Now I don't mean "Killer GM" fun, just a minor tweak or two to catch the players offside a bit.
You know.....like if you players always choose to take right turns when starting out a dungeon, you can work with that.
I know as a player that when the GM throws down a map, things just got "real". To this end I like to sometimes bring out map and/or prop pieces for no reason at all. Sure it can "waste" a little bit of table time, but when you put the players on alert when they aren't expecting it and then something doesn't happen..., well that is a teaching moment.
Normally in a tournament your encounters are somewhat spaced a bit. Throw down a map and the players are expecting to complete some challenge (combat, social, or trap) and move on. With this map I combined two encounters to help throw off the players just a wee bit.
On this map the players are travelling from right to left in an underground tunnel. The tunnel opens up and the players can make out a couple of things: a small pool of water with a post and rope leading into the water, a small tunnel at the back, and an awful stench near the entrance to the tunnel.
As the party checks things out they get attacked, which is the combat encounter. I used Stirges so the party would be engaged/grappled and not try to flee into the next encounter. When the battle was done the party has two options: try the water route or go along the side tunnel. The distance between the two pools was such that and average swimmer could just make it.
The stench? That is an olfactory marker for the inhabitant further along the cave system to know not to take the path. In the path are four Shriekers which cause such a ruckus that everyone knows there is someone at that spot. Setting off the Shriekers turns the next encounter from a combat where everyone can be surprised to an ambush that can really put the party at a disadvantage.
The players were only shown the right side of the map and they assumed that the encounter was pretty much done after defeating the Stirges......