I once saw someone trying to do this. On the face of it, it seemed quite a logical idea: Some dirt had gotten into his tank, and it was rattling around when he drove, or something. He'd siphoned all the petrol out of the tank, so there should just be dirt left. He couldn't cut the tank open, so he shoved his vacuum-cleaner nozzle in through the filler pipe, to clean it out.
At this point, I should explain that it was a separate-unit style vacuum cleaner, not an upright, and it wasn't the newest in the world. Let me explain for a moment how the vacuum cleaner worked:
Dirt gets sucked into the nozzle, and pulled along the bendy pipe, then it comes to the main unit. There's a bag with a filter in it. The air gets sucked through the filter, but the dirt is trapped, and fills up the bag. Just past the filter, there's a big fan attached to a motor. This provides all the suck. Then at the back, there are some vents for the air to escape. Here's a diagram-type-thing:
Nozzle < Pipe < Filter < Motor < Exhaust
What he hadn't anticipated was the motor: It occasionally sparked a little. This was normal for vacuum cleaners at the time; motors weren't as refined as they are nowadays. In any other situation, the sparking wouldn't have made any difference. It was all internal, and there was practically no risk.
The problem? Petrol vapour. He checked the nozzle was in the filler pipe, and switched it on. The petrol vapour (Which is a gas) was sucked into the nozzle, along the pipe and into the bag. Being a gas, it wasn't trapped by the filter, and it got sucked in, past the motor. The motor that sparked.
As the stream of petrol vapour (Which, by the way, is highly inflammable) gushed past the motor, a spark came from the motor, and ignited the petrol vapour. The vapour then carried on out the back air vent. On fire.
Flame-thrower!
A big jet of flame shot out the back of the vacuum cleaner. Fortunately, this melted the fan, and stopped anything worse happening. But it was still a bit of a shock when it happened.
So don't do this. Seriously, don't.
SharQ says: All electrical motors spark a little, don't they? But you are indeed right.. It sounds like a VERY bad thing
lj says to be really pedantic, the newest dysons have an induction motor, and so (in theory) shouldn't make a spark, as they have no commutator. I wouldn't like to try it though, especially with a £400 vacuum cleaner. There's a similar (and probably apocryphal) tale, of the techie that tried to hoover up a toner spill with a dustbuster. Particles of toner are (apparently) too small to be caught in a hoover bag, and so shoot out the back of the dustbuster to much the same effect as the car/vacuum cleaner/flamethrower.
strawberry says "This provides all the suck." -- god, what a great quote! | 
