My all-time favorite one was when the PCs where tasked by a high level cleric of Pelor to rid a former garrison of a Master Vampire and his minions.
The PCs manage to fight their way past all the low minions and finally confronts the BBEG's right hand man - a brute of a vampire with a two-handed sword, lots of strength, and power attack-related feats. They manage to hit the brute with a Bokken of Distruption, and he fails his Fort save - dies instantly.
However, the PCs are pretty weak, so they retreat to rest and heal.
Two days later, they arrive back to take on the Master Vampire - only to find that he resurrected his right hand man, and turned into a Vampire AGAIN! Sure, the henchman was a full level lower than the first time, but the fight didn't go as well this time around. The brute attacked and destroyed the Bokken of Displacement right off the bat (pun intended). The PCs eventually escaped, though with serious losses and without destroying the BBEG.
Good times.
Peterson