I loved this mode in Mega Man 10, even though I'm so bad at it (compared to the rest of the Internet). In a way, Balloon Fighter had an Endless mode as well (the Balloon Trip), but the fact that everything after the first part is randomized makes it very difficult. Now, in MM9/10, there were several segments (often resembling areas from previous Mega Man games) with teleporters at the start and finish. The order in which they appeared was randomized. They also both had absolutely brilliant music.