Ok, lots of things.

-Put the finishing touches to the new game over system. Which, in turn,
allows for...
-Extra lives! Yeap, fully working, with new pickup and all that. You're
welcome.
-Changed the player to spawn with a short invincibility (for the extra
lives thing) and in the vertical center for aesthetics.
-Fixed a minor graphical glitch with the homing missile (the target
sprite was visible in the upper-left corner before moving to an actual
target).
-Fixed some enemies crashing that I missed from the previous change in
their behavior.