Turrican 3/Mega Turrican
In the code of Mega Turrican, sprites representing what appear to be Mario and Sonic have been found in the tiles of the first stage, but unused anywhere in the game.
Mario appears similar to his Super Mario World sprite, while Sonic appears based on his look from the 8-bit games. They both appear seemingly frozen in carbonite, like Han Solo in the Star Wars series. Not being licensed, however, these can't be considered the actual Mario and Sonic, but rather characters based on them.
The possible use of these sprites was found in the game's development diary in German magazine Amiga Games, that showed them used shortly after the battle against the first miniboss of the game.
Turrican's company Rainbow Arts held no right over Nintendo's Mario and Sega's Sonic, but they ultimately removed any reference to those series.