Heat is a big cause of problems...gets the moisture moving out of the paper (or carbon tissue) and into contact with the emulsion of the negative.
For both carbon and platinum printing I go with the straight camera neg on the paper/tissue...and yes, I have lost a few negatives over the last 30 years with carbon printing learning what to do and not to do to keep it from happening. I use 750W merc vapor bulbs and have a fan blowing across the glass of the contact frame, for example. I have never an issue with platinum, but then I use to over-dry the paper. Now I just air dry, but still no problems.
The greatest danger I have come across has not been chemical contamination, but ferriotyping (bad spelling), or the creation of areas of high gloss on the emulsion side of the negative caused by pressure, heat, and a little moisture. In areas of even tonality, these areas show up as they pass light a little differently.