Secret mystery game(s)

[termis]

Hmm... That's a good quiz...

Night Trap for the Sega CD?

[Black Tiger]

Urusei Yatsura - "Stay With You" was sold with an extra audio disc included in the case. There's also another version sold without it. Trying to think if there are any others...

I think that the LE Kabukiden might've had an audio CD as well. 3x3 Eyes came with a VHS tape...

[runinruder]

I guess Kabuki Den is a good game to get as far as surprise inclusions go.  When I received mine a few years ago, it was accompanied by the Fighting Street disc.  Although I suppose the joke was on me, since Fighting Street is a disaster. 

[Black Tiger]

I guess Kabuki Den is a good game to get as far as surprise inclusions go.

Apparently not if you're looking to play Kabukiden. 

[FM-77]

You can never get enough Ys games.