Well, the most recent prank I pulled was on my suitemates. They were coming home late everyday after watching scary movies with their friends and waking us up in the middle of night with their screaming so my roommate (mostly I) and I decided we had to do something.
I took red paint and wrote "Where have you been??!" on their window in bloody drip font style. Then I left the blinds up (it was night time), opened the tiny window because it was windy outside and taped a white piece of cloth with red splotches of paint on it from the window so it would blow into the room all creepy like. I covered the light switch with cardboard because the effect of the paint looked fake when the light was on and left their door closed and the main room light off. Also, I downloaded a creepy ringtone and left my phone in one of their beds.
When they came in and couldn't turn on the light, they were like "What the hell?" I had my roommate dial my number, so the creepy ringtone started and one of them was like "OMGOMG where the music coming from?" and then the opened the door fully and "What the hell?"ed at the stuff on the window. The sensible one ripped the cardboard cover off the light switch and turned on the light, but by that time, my roommate and I were stuffing our faces into our pillows and laughing our heads off.
It was definitely worth it.
They came into our room to tell us it wasn't funny, but we pretended to be asleep. They were like "We'll get you back."
So the next night, I was the last to shower, so I took some red paint and wrote on the wall of our standing shower. "Help meee..."
Unfortunately one of them woke up earlier than usual and was so tired so she didn't notice and just took a shower. Then, she got out and saw the paint splattered everywhere (because the water from her shower mixed up the words) and screamed. The other went in and was like, "OMG what died in here"
I prefer giving other people bad ideas and keeping my hands clean if I'm in a mischievous mood. I did park calls when I was a middle schooler, back in the day before caller ID. Would use my "sexy" voice and leave messages on people's answering machines.... Then my friend accidentally gave me my aunt & uncle's number and I was so freaked out that I've been an angel ever since.