I got an error while fighting the first boss, and again on a later one :( Also I think I lost my save file, because after I restarted the game I had to play from the beginning..
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FATAL ERROR in
action number 1
of Step EventFirstBossAttack
for object Smash:
Variable SmashBounceBack.checkX(100012, -2147483648) not set before reading it.
at gml_Object_Smash_Collision_262
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stack frame is
gml_Object_Smash_Collision_262 (line -1)