Also, one thing I did to mess with my players:
I had the PCs mistakenly replace a mayor of a small town with a doppelganger. The plan was perfect, and worked without a hitch, which was surprising given my reputation for twisting plots.
The doppelganger had started by making a deal with one of the mayor's staff, who didn't have a name, but we will call him "Fred" to avoid confusion.
The doppelganger provided Fred with daily doses of drugs to put into the mayor's food, which would ake him forgetful and impatient. This seemed odd to those who worked closely with the mayor, seeing him forget the names and faces of old friends, and suspicions were raised, but nothing overt had happened yet. This went on for a couple of weeks. Enter the PCs, with a caravan, who meet the mayor and see him unable to remember the caravan leader, who has been making trade runs through this town for years. They also notice that the mayor wears strong purfume to cover the subtler smell of alcohol (one of the PCs had scent), which was just a convenient quirk of the man.
The doppelganger, recognizing the inn as the most likely place for out-of-towners to be passing through, had Fred tie him up in a room adjacent to the PCs' room. He (the doppelganger) was dressed in clothing owned by the mayor, wearing the same purfume, but a weaker dose to make it appear it had been days since the last application, and stank of not having bathed for a few days. He got the PCs' attention by knocking on the wall after Fred had left discreetly. He also had Nystul's magic aura cast on him to negate detect magic, while Fred had laced some of the mayor's clothing with illusion magic (harmless unnoticable without magical detection). The players were suspicious (again, I was known to be the bait-and-switch, "save the dragon from the princess" kind of DM), but as they used their amateur detective skills, everything seemed to check out. They confronted the "impostor" in his room, and had him taken to prison by security, while the "kidnapped mayor" thanked them and "resumed" his duties. It was not until after the PCs had left the town that I revealed to them what had actually happened in the form of a letter. Apparently, the doppelganger just wanted to embezzle the sales tax money from the town (the town boasted one tourist attraction - the World's Biggest Brothel) and leave. Fred confessed under pressure, and out of spite for not getting his promised cut, but the campaign ended before the PCs could right that wrong.
I wasn't sure how I wanted the situation to play out, but I wanted to see if the scheme would work in a world where doppelganger kidnappings/replacements were a normal problem.