My car is a diesel, but if I picked up the wrong petrol hose at the garage the nozzle would not fit into the petrol tank, so then I would realize that it's the wrong one. Also most petrol pump hoses for diesel are a green colour.
I nearly did it myself a couple of months back while in a hire car. I've had diesels for years so just subconsciously pulled up and picked up the diesel pump. Luckily the nozzle was too fat so I realised before I could start filling it up. Disaster averted.
Done it twice in last 5 years. Put unleaded in my car when I first got it about 4 years ago and then some how out Deisel in my Wifes unleaded car a couple of years later. Got quite a bit of hammer for it from mates lol. My mates garrage only charged 80 quid so I saved on what AA wanted to charge me
I did it in Adam's Merc once. At the time I was driving 4 different cars regularly (mix of diesel/petrol/manual/auto) and got confused. Luckily I only put about £2.50 of unleaded in before it clicked. I just filled tank right up with diesel and got away with it fortunately. I was only meant to be putting £20 in though and it ended up costing me about £60 to fill it up.
get one of these alerts Kevb I did the same thing many years ago and again about a year ago. I heard about this and got one http://www.amazon.co.uk/Think-Diese...qid=1434295268&sr=8-1&keywords=diesel+warning and this too http://www.amazon.co.uk/Fuelrite-wa...qid=1434295268&sr=8-2&keywords=diesel+warning they work a treat.