All known bugs in Super Space Acer are resolved. Trickiest one had a single instance of a helicopter enemy just randomly popping into place near the middle of the screen. Turns out I had an off-by-one that allowed the random speed generator to generate a -128 (due to signed char wraparound), and it happened exactly once in the generated patterns, in the middle of the very last stage. ;)
Now I just need to play through all the modes in normal and F18A modes and look for anything else weird. ;)