The last time I played a joke on my gf it also went wrong...
We had been watching this thing on TV (4 part ITV drama series about a haunted house where a little girl called Amy drowned - various flashbacks and creepy whispering of the words Amy etc). My lady gets quite scared at scarey stuff like that.
Anyway she went to the toilet half way through and when she came back into the room I hid behind the door and she must have thought I was downstairs. As she walked right by the door into the room I whispered "Aaaammyyyy" right into her ear shot. She jumped and put her hands up to her face and pushed me away making a horrible face like she'd just been stabbed. She could barely speak or catch her breath then put her head in her hands and cried for about 10 minutes. I initially LOL'd and then felt awful comforting her for the rest of the evening. The TV went off suffice to say!
Women.
LOL - thats legend.