The only verified incidents I've come across have been the result of a label side scratch. The label side seems to contain fewer layers than the playing side, and is thinner as a result. Scratches on this side can expose the aluminium surface, which then happily oxidises. The BBC used to teach operators to handle the discs by centre and edge only just like vinyl, and always return them to their cases. Don't pile discs one on top of the other. Other problems can be caused if the disc has a colour printed label surface.
I have a copy of a notorious edition of The Terminator on DVD, for which the company actually had to arrange for free returns and replacements. The label inks migrated and crept round to the playing surface, causing the discs to be sticky to the touch and unplayable. I never got round to sending mine back, but found out that you could just wash the whole disc with washing up liquid and lo! it playeth! Mind you, I've not tried that recently - it may be in rag order in the box now.
