Most single sided Japanese discs have a short video that says "please turn the disc over".

Were there ever any with hidden stuff on the supposedly blank side? Like bonus material.

I heard that some discs were recycled from single sided ones that failed QA, but do they ever have anything on the supposedly blank side, or is it erased somehow?

Check my post and look under wrong side bumpers.

Thanks. All the images and videos are broken in the UK, but I'll look tomorrow with a VPN.

I don't follow why you need a top loader in that thread.

Actually, someone else made that topic using a YouTube video that's been removed for, like, a year or two now.

YouTube does hate media preservation...

I have seen the blue screens before. I'm wondering if any discs have hidden content related to the main content though, or stuff that was accidentally left on there.

Hate to go off-topic, but I don't think that's true about YouTube hating media preservation. Look at this article that just came up.

The problem is that their copyright dispute system is completely, and utterly broken.

I think it was mentioned there. The big difference with top-loading/tube-laser players is that they are relatively "dumb" compared to later players. They can and do use "analog methods" to read disc content and adjust/align their motors and tracking systems to lock onto the disc's content. These methods open the potential to "fool" the players or otherwise scan/view areas on discs that are not part of the intended active program areas.

Later players, in contrast, depend on/require the encoded "digital" data (just the frame/time/chapter information, or on later discs the "table of contents" data) to control their tracking systems, and they only "allow" user operation/control of player functions within/subject to that digital data. (Working this way makes player operation and normal disc-watching "smoother/nicer", at the expense of the "playing around" that is discussed here.)

I am sure you are already familiar with them, but in terms of "content on 'blank' sides", the majority of Discovision titles in the first two-plus years of the format's availability had a "blank" or "dead" side that was indeed a "recycled", otherwise-unwanted (at the factory) leftover/failed manufactured disc side. There is a lot of information about those on this forum and elsewhere.

Since I'm doing DdD captures of discs, I wish I could capture the "blank" sides, but my player is a tray based one and won't read those bits.