Constant Vigilance!
After having it drummed into him for almost 3 years now, it was really no surprise that he finally got it. The last few years of fighting death eaters and various dark creatures at every turn had also served to make him slightly trigger happy with his wand. So when Harry Potter, now turned 17, entered the kitchen of number 12 Grimmauld Place and was bombarded by sound and movement, it really shouldn't have been a surprise how he reacted.
Upon shouts of "SURPRISE!" and "HAPPY BIRTHDAY" he jumped backwards, drew his wand and fired of several hexes before he even realised what he was doing. Of course by then it was too late. Ron and Hermione were frozen on the spot, Fred and George were being attacked by bogey bats, and poor Lupin had been banished into the back wall.
After quickly unfreezing his two best friends and apologising rapidly, harry glanced sheepishly at his feet.
"What the bloody hell possessed you to do that!" yelled Ron.
"Um... uh... Constant Vigilance?"
AN:-pretty bad i know but it just came to me, wrote the whole thing in about 2 and a half mins, that's my only excuse for why it's so bad lol. Please review, even if they're crtisising they still make my day!