What if you had planned a really cool encounter in an old destroyed temple, that included random nasty steam vents and an eight headed pyro-hydra and.... your players ran right over it?
The characters in our main game are 7th level. The have access to all kinds of buffs when time allows. As soon as they saw the inside of this nasty temple, but before they knew about the hydra, they went about buffing themselves up. Enlarge person, bull's strength, mage armor, shield, etc. After a few rounds of buffing they wandered in. A few more rounds later the beasty shows its faces. Smugly they finish the buffing with prayer and haste, come get us beasty! Well the hydra gets just outside melee range and unloads eight heads of fire breath. Large chucks of character hit points and the smug looks evaporate. A little panic starts to set in (DM refrains from gloating) and the two heavy melee fights decide it is do or die so they make their move. Lots of attacks due to level, full attack option and haste. But the dice, rather than turn treacherous on them, does just the opposite. Not only is every attack a hit, three are critical hits, and the damage dice are smiling on them just as broadly as the d20s. The results are impressive and statistically improbable. My nasty eight headed pyro-hydra lies on the floor of the temple in pieces, rather than hovering over the panic stricken party.
Where was the terror? Where was the anguished cries of help to their deities? Oh, the inhumanity!
They did plan. They did work together. They did make good choices. They did roll very, very well.
What if you had planned a really cool encounter and your players ran right over it? After I got over being stunned, I congratulated them. They earned it. It was time to allow them to bask in their glory. Not to worry, there are more encounters waiting and you just never know what the dice will say next time. You see, the dice never lie.