I took my car in for a service and as part of it they topped up the coolant, unfortunately the halfwit that did the job put an incompatible coolant in (I had green, he used red) resulting in the mess below and an overheated engine. Fortunately I had water in the car so I didn't end up with omghgf or any damage (although I did get a bit messy when I removed the radiator cap) this also managed to blast quite a bit of the gunk out. The heater was also pretty pathetic post clogsville (was like an asthmatic ant blowing through a straw after eating a polo mint)
Took back to the garage and got a full flush and replacement of coolant, although took a bit of internet persuasion to the desk idiot to prove that the coolants were incompatible.
Mind you he also tried to say that the fan wasn't working and as we were stood in front of the car it cut in, then he tried the old "well that's at the high end of its temperature range"