Some days ago a customer asked the store where I work to swap their PC components into a new case. This happens from time to time as people find a new case they really wont over the one they have.
However this was done by the customer as the PC was either in or next to a kitchen - im guessing in a restaurant due the amount of grime that had built up over time (see pictures below).
As you can see it looks like the insides of a heavy smokers PC, but the smell was definatly kitchen... and everything was coated with a thin layer of cooking oil. The cooking oil had helped to glue the dust together.