A couple years ago I made a big project to rip all my DVDs.
Out of several hundred movies only 6 were unplayable. There didn't seem to be a pattern to it either; age of the disc, wear or handling, big budget then current release or old movie slapped onto a disc in one of those cheap cardboard sleeves.
Out of my collection of TV shows on DVD, easily a quarter of the discs failed, and if one disc in a season of a show didn't work most of them probably wouldn't. Many had visible blotch marks in them. I figure they probably used a cheaper manufacturing process for TV shows where they were selling 3 to 6 discs rather than one, maybe two discs with a single movie on it.
One of my first jobs in IT I worked in a local newspaper - I thought I wanted to be a journalist turns out it's boring. Anyway we had all the old archived papers on a dvd and someone used it as a coaster and erased about 10 years worth of files. Naturally there were zero backups, so that data was just gone. Fantastic.
Fortunately the local library has backups but they're on microfiche, so not particularly convenient. I think Google might have scanned them now though so they're probably archived again.