Different versions of Sonic 1
In Sonic Mega Collection, the version of Sonic 1 you play is the Japanese version.
You can play the US version by pressing, on the info screen for Sonic 1, Up, Z, Down, Z, Left, Z, Right, Z. The American version differs in that the clouds don't move in Green Hill, there's no wavy water in Labyrinth, and the level select is in the wrong order.
To play another Japanese version, which fixes the bug which makes you continue to take hits when you hit spikes even if you're flashing and supposed to be temporarily invincible, hit, on the same screen, Z, Z, Z, Z, Z, Z, Z, Z, Z, Up, Down, Left, Right.