I don't know about SQ 4 (it's the SQ that I played the least, other than 6), but I remember seeing two minor bugs in the first two games. In SQ1 EGA, if you take the cartridge after seeing it at the console on Kerona, it still says that it's in the slot even though it's in your inventory. In SQ 2, in the famous plunger hanger part, if you let go of the plunger before the sequence is done, the walls rise and the door opens, but the floor still moves and you can't escape.
That's just the inner game tester in me still thriving even though I haven't tested a game professionally for a year! :D
Till next time stay cool B)