HAR, My water broke while undergoing the Group B strep test a few weeks before my due date, on my Gyn's table. I didn't know it had broken, i figured i was just gushing w/ all the extra (ahem) juice i, as pregnant lady, had been producing, and was SO EMBARRASSED i even said "um, i think there might be a problem down there w/ excess leukoreah right now", my face in flames bec. my reg dr. gone, i had some man dr. (i'm a gal-gyno-goer), man dr. (who was awesome by the way) was like, um, okay just move toward me a little" as he was snapping on the rubber gloves and furrowing his eyebrows as i'm sure even as a doctor the word leukorreah (like episiotomy) gave him the willies as much as it did me. I moved maybe a 1/4 inch and it was a gusher, all down the front of the table, the floor, soaked the little paper thingy they give you to cover with! I looked at him laughing, i just couldn't not laugh, he said "that's not just any fluid! your water just broke!", took off the gloves and directed me to the ladies washroom. Ended up driving self to Central Dupg still laughing, where I parked at Valet, told the guy I was in labor and he shouldn't prob sit down on my car seat w/o newspaper or something, he said "oh this happens all the time" as he sat down on the little blue cottony waterproof thing my dr. gave me and got his buns soaked. THAT HAD TO BE maybe one of the more unpleasant days in that kid's life. I gave him $10 tip (all the money i had), and a more valuable tip -- to listen to pregnant ladies when they say don't sit in my seat! :) My 2nd child's water also broke, at 1230 at night, I felt a sharp pop when i lay back down after my midnight pee, and the labor pains started immediately after that, about 6 mins apart. I am pretty sure her head blocked the leak tho, as there wasn't too much fluid, a much less dramatic entry into the world.
All said, it's a good idea to get waterproof mattress pad... I can't say that I could "feel" that the water was ready to break on either occasion, but each time it did there was ENORMOUS relief.