I've read quite a few threads on other message boards on the topic of DVD rot. Apparently a number of Anchor Bay DVD have had this problem--a manufacturing defect apparently--that will allow a disc to play fine for a while, then inexplicably stop playing. Sometime a brown coffee-like stain is reported on the disc.
In addition to the Heathers and Opera discs, FRANKENSTEIN CREATED WOMAN and THE WITCHES have also been reported as having this problem. If you have any problems, you can go here to report it:
http://www.theregangroup.com/pages/dvdhelp.php