In a d20 Modern campaign I had once, one player went out of his way to bludgeon us with his knowledge of hospitals. He worked as a computer technician in a very large hospital, and in two different adventures they went into a hospital. Not being an expert on the subject, and it being a fairly cinematic game, I went with the way hospitals are always portrayed in action movies and TV shows. When the PC's went to look at the charts on a person they were there to see, he pipes up on how recent legislation into medical privacy made it illegal to have those charts in the open, so they couldn't just walk up and view his charts. I told him that it was my game, and this isn't the real world, it's like an action movie or a TV show, and you see it in the movies and TV all the time, he got really upset by that saying that if it's unrealistic there is no way he can play his character. In a second adventure, the PC's decided to break into a doctors computer to steal some records. He took this opportunity to nitpick everything I said and he did, since he worked on computers and medical records. Everything the PC's did, he said wasn't possible because he knew how computers like that were set up, and every ruling I made he said I was wrong because he knew the subject better than me.
As a sort of reverse to the whole issue:
In D&D, my friends and I had always interpreted Fireball as being just that, a big dang ball of fire that burns everything in it's path. It sets buildings on fire, and has lots of collateral damage potential. A new player joined our game, and when he tried to cast fireball at a wagon full of hay to get the archers on top of the haystack, and I told him that not only were the archers dead, but the wagon was singed and smoking, and the hay was practically ashes. He then threw a fit, saying that a fireball is a flash-fire that doesn't hurt objects, it only hurts creatures, and the rules were very clear that a Fireball doesn't set things on fire since it lasts too short a time. I told him that any fire hot enough to do 10d6 damage, 35 on average, which can reduce practically any townsfolk to instantly dead even if they make their saving throw is going to ignite hay and leave scorch marks on the wood at least. He then started to pull out his PHB and sift through to cite the rule to me. I told him that he was new to this game, we've been playing like this for years, and I'm just as likely to pull out the old 2e Spells & Magic which encouraged DM's to use collateral damage for spells. He was very upset that I wasn't following the RAW, but the rest of the group really liked the idea of spells causing damage to their environments, since it seemed silly to most of us that throwing around a fireball in an urban environment would only kill creatures with HP and ignore all the inanimate objects in the area.