Anyone know which fic this is from?
Harry and Hermione know about the magical world way sooner (can't remember if it's because of time travel or getting into Diagon early) and developed a Mad Eye Moody level of paranoia, especially since they know the death eaters escaped being sent to prison by claiming imperius curse. Due to this the two are CONSTANTLY testing for hexes, traps, spells and potions (McGonagall is beyond exasperated with them where as Moody when he finally meets them is seriously impressed). Due to the constant tests they find out that the drinks during the opening feast (specifically the pumpkin juice) is laced with potions. The causes a massive outrage when they stand up mid meal and shout the drinks are potioned which prompts the upper years to cast detection spells and a small riot nearly forms. Snape is initially pissed at them but eventually everything gets sorted out with the house elves getting everyone new drinks. Later on Snape takes a flushing drought just in case and is horrified with his actions from his own 5th year and onwards before realising he had been potioned for years.
This also has a negative effect of fixing a bunch of damage to his features the potions caused making him to "handsome" which to his horror causes him to gain a rather enthusiastic fanclub so he had to get McGonagall handle his detentions because he's frankly terrified of them.