Do you mean a 2D player in a system with a 3D-capable AVR and 3D-capable display?
Or a 2D player in a 2D system?
I was under the impression that many 3D discs include code which checks for 3D support in all of the equipment involved and refuse to play if they all aren't 3D compatible. I've certainly seen complaints posted in the forum by people trying to play some 3D discs which refused to play in 3D systems because the discs thought something wasn't compatible.
(I have a 2D system. The only 3D disc I have is in a John Carter combo pack. I could test it this evening if nobody else provides any examples.)
On multi-disc sets, with the 3D and 2D versions on separate discs, the 3D disc will not play on a 2D setup. They will show a message saying that you need a 3D TV and capable player. I don't have this but the FIFA Soccer disc says "Only plays in 3D" on the front cover, and I've heard the set of 3 Toy Story movies only plays in 3D also.
I just tried the John Carter 3D disc. It displayed the Disney castle, then prompted for a language, then showed the error message "The disc has recognized that your player and/or display does not support 3D playback..."
Yes- In almost every situation when a 3-D Bluray is also paired up with a separate bluray disc with the 2-D version.
With MVC Bluray encoding all content could (and by default "should") be able to play back fine on 2-D gear. This restriction is an authoring decision.
The studios often make the 2-D bluray with the special features and then put the more bit-rate intensive 3-D version on its own disc. My guess is with these situations the studios don't want consumers immediately selling off their extra copies (be they 2D or 3D) .. hence the authoring restriction. Its worth noting I once tried playing my "Tron Legacy" 3-D Bluray disc in my 3-D Bluray player when it was hooked up to a 2-D projector. Playback was still prohibited.
I agree it is an authoring restriction. When I order a multidisk title, I often give the 2D BD to my daughter. I'm aware that I lose the extra features but usually, I watch these once and never again, so the disk has no additional value to me.
I do recall once experimenting with my Sony 90ES projector and my Vizio 32" panel and here I can shut off the 3D and watch in 2D if I wanted to. But in each case the player is a 3D player.