Just the Usual...
The only contaminants I can recall are the usual beverages (Coke, Tango, coffee, etc.) and stationery.
Sugary drinks were always the worst, in the days when keyboards cost a lot (£100+ sometimes) we'd end up dismantling them and scrubbing the boards under the tap. They usually survived to work again.
Many years ago when the first plain paper fax machines came out we had an FD who insisted we reused paper in it to save money. Which was all very well until someone put a piece of paper with still wet Typex on the top of the stack the "wrong" way up. The £100 or so for a new toner/drum kit after I'd tried every trick in the book to revive the damaged one would have bought a lot of paper. Reusing label sheets could have a similar effect when a label peeled of onto the drum but 1,1,1 Trich would usually get things clean again.
The current employers had bought two very expensive sheet feed HP scanners. One of these wasn't feeding properly which may have had something to do with the paper clips and staples that had been ground through the mechanism. It didn't survive the experience!