Life is, in a manner of speaking, all about learning lessons. Some of these lessons we learn the easy way - it is taught to us directly, gently, and in a manner we're comfortable with - and some of these lessons we learn the hard way - fire is hot, for example. This is no different when it comes to GMing and Playing in table top RPGs.
Today I'd like to hear about some of the lessons you've learned the hard way as you've gone through your various games. Was it a positive lesson? A negative lesson?
For me I figure I'll stick with a fun but basic one. I learned, the hard way, that try as you might an early and dramatic reveal of a big villain will often end with that villain dead or wiping the party.
The scenario was simple enough, an AD&D game that I ran for some friends on weekends. I had this great and original idea (super original, I can't emphasize the originality enough) that the wizard who would appear to be helping the PCs was actually one of the big bads. However, I wanted this to be an awesome reveal. One where I could point at things later on and show how the wizard had duped them. Only, I never got that far and I'm really not THAT much smarter than my friends to pull a con like that off with little prep time for real engagements.
So the first meeting with the wizard happens and sure enough the Paladin more or less starts things off with Detect Evil. Now the wizard is evil but I figure he needs a way around that (I figured the Paladin would do that, because why wouldn't you?) and gave him a charm that prevented alignment detection. I wasn't prepared for, after spending a few hours with the wizard, the party spell slinger to cast detect alignment. Baffled I said the wizard didn't really have a detectable alignment (the charm warded the spell as opposed to lying about alignment which would have been smarter.) The PC nods, gets the players together, and says he thinks the wizard is up to something because why else would a good wizard hide his alignment from the heroes he is looking to help?
One thing led to another, and I was also taught how quickly a group of PCs could dismantle a wizard when they caught him by surprise before he prepped his daily spells.
Looking back it was a good lesson to learn (be faster on your feet, don't assume you can outcon 5 people who know you, and have a plan in mind if you need/want an NPC alive after meeting the PCs) but at the time it was pretty much the end of the session for the night..and the next night...as I had to completely re-think the whole adventure.