I was cabin crew once on an MD80 from KITH to KELM - a 20min hop where we weren't even to go above FL100. Well we got catered in KITH through the 2L door, and catering didn't close it properly. Apparently a sealing strip was folded under the door? Well on climb out I got a call from the crew member at 2L 'Michael - this door's making a hell of a row!' When we leveled off I went back there, and sure enough this thing was howling, and it was cold!
I went on to the flight deck (this was pre 9/11 when we too were allowed into the hallowed inner sanctum for reasons other than "flying the plane while the pilot takes a poo"
) and they were both looking up at this gauge wondering why they couldn't pressurize the cabin.
Needless to say, it was such a short flight we just carried on, landed safely at KELM, and closed 2L properly!