First - MiG weathering powders. Open the pot, and about a quids worth of the contents leap out all over my jeans. Trust me, cleaning this stuff up is a nightmare. You have to wash it out of the clothes, the floor, the bottom of your shoes and pretty much everywhere else.
Horrid, horrid stuff.
Second - Electrostatic grass and the lose top on my Flock-it tool. Wave it over the area to be grassed and the whole lot dumps itself on the model. At least some quick work with the vacuum cleaner, holding a handkerchief over the end to save the fibres, sorted this out.