A mixed mode CD contains data + audio tracks stored within a single session. If you tried to play a mixed mode CD on an old CD player you had to skip over the data track (usually track 1) to get to the audio tracks. Newer CD players can usually recognise the CD-ROM content and skip over it automatically.

Many of the mixed mode CDs we have in the database are video games such as Quake.

They were also used by some artists to include multimedia content on their audio CDs, and by some magazines for their "cover discs", see Promúsica CD 27: Abril 1999

There's quite a few in the database, hundreds, maybe close to a thousand.
Mixed Mode CD-ROMs collection