So I've been toying around with the game a bit to see if I can find new weird ways to mess it up. I haven't found anything that's viable for any current routes, but some interesting stuff nonetheless. This was all on HD but I'm pretty sure it'll work the same way in Standard as well.

So basically what I've been looking into is abusing the game over scenario. When you run out of lives, the game gives you 10 seconds to continue. My hypothesis was, if I tried forcing a continue screen during scenarios that require the alien, I might find some interesting interactions. Here are my findings:

1-1: School bus picks up alien - I shot the install ball as he started his jump and dug underground to kill myself. This made it so that I could die shortly after the boss. The result of this was, the bus passes the alien and keeps moving across the screen. I'm not sure if this actually saves time, and if it does it's basically negligible. I don't see any case where this trick would come in useful unless you're already in a bad situation.

1-2: Fat Kid picks up alien - I jumped into the street at the same time that I killed the bee robot. Not much different here, it just cancels/delays the Fat Kid flying in to pick you up, but the level still ends at the same time. No time save.

1-5: Level end - By far the most interesting I found. Since this doesn't have the long level end song it meant that I could potentially end the level on the continue screen. What I did was kill the final robot at 0 lives just as he was spawning a rocket, so that it could kill me just before the cutscene starts. Because of this, the results screen was skipped entirely, but when the next level started both players showed the "Please Wait" message and Boris hung on his starting animation. I'm assuming the game will hang here forever.

2-4: 2nd train boss - This one I actually knew about before and is the reason I started wondering about these interactions. Basically, at the start of the second boss in 2-4 there's a cutscene where Boris unhitches the train. If you die just before this cutscene (0 lives isn't necessary if your timing is good), you will respawn during the cutscene, which will let you stand on the box and charge up a shot before the boss starts (something you don't have the opportunity to do normally). This doesn't save a ton of time but it helps if you don't have a powerup when you reach the boss. I'm not 100% certain if it works the same way in Standard but I'm assuming it does.

3-4: Level end - I killed the boss and let myself get killed by a bomb, spawning the continue cutscene. This one's similar to 1-5 in that after the Install Ball jumped out of the robot, it triggered the cutscene where the alien is supposed to walk towards the Install Ball. Since there was no alien, it gave me the 2 "Please wait" messages and hung there forever.

While there's nothing here so far to help with speedruns, it's still pretty interesting. As far as I know, these are the first two reproducible hangs discovered, though I might be wrong.

Other interactions that I haven't tried yet:

Normal level end: What I want to do here is basically end the level on the continue screen to force a scenario where neither player exists in the next level. I've tested it with 1-5 to 2-1, but 2-1 has a cutscene that depends on the alien. I want to see what happens if the actual level starts and neither alien is on the field. Unfortunately, this is very difficult/impossible to do because most levels end with the alien cheer + song which is about or a little over 10 seconds long.

1-3 level end: For this one I specifically want to find out what will happen with the cutscene at the beginning of 1-4 if there are no aliens. I'm assuming it'll be a hang like the other levels I've tried.

2-5 level end: This one I'm very interested in seeing. I'm assuming once it cuts to the alien ship it'll hang there, but I want to see it for myself. I think it's very hard to do as you'll basically have to get hit with a bullet around the same time as you kill Boris.

2-6 level end: Haven't tried yet, but I'm assuming it works the same way as 1-5. Interested to see what happens with the truck cutscene in 3-1.

3-2 level end: I don't think anything is possible here but with how the level ends so differently from other levels it might be worth trying some experiments.

I have some videos of the ones I've experimented with already and I'll upload them later.

1-5 to 2-1 would be interesting in standard if it works, as for 2-6 to 3-1. Since results screens are much longer in standard, it could be used in speedruns by exiting after the level starts. I don't know if it would work or if it would save much time, but it would be interesting.

just tried this, like months later, standard just gets a green screen (softlock)