I used to work for a company that used standard desktop PCs in sealed enclosures as controllers for process control equipment in (primarily) automobile factories and steel mills. While the enclosures were well sealed and met all the applicable NEMA standards, there were always cases where the customer would leave the door open for days at a time, or simply never close them up in the first place.
I've seen computers that had transitioned gradually from being air-cooled to being oil-coooled, by virtue of the oil vapor condensing out of the air and filling the entire case . I've also seen at least one computer that was entirely filled with lint from a textile operation of some sort - when I opened up the case, you couldn't see anything inside it at all - it just looked like a featureless gray blob.
I've also seen failures due to people welding on the enclosure and accidentally switching the "hot" and ground leads on the welder. Basically, the insulation melted off every single wire in the entire cabinet, and pooled in the bottom as a multicolored puddle of vinyl.