Another one for ya. This happened during Endless mode with Mari selected, inside the Shop. Every NPC spot had spawned the special Blue Ant for the achievement. I interacted with them once, the NPCs stayed while the enemy got sent to jesus at mach fuck. I interacted with the NPC again out of curiousity a couple of times and, viola;
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ERROR in action number 1
of Step Event0 for object o_player:
Variable o_npc_cameo_v2.q_entry_toggle(100593, -2147483648) not set before reading it.
at gml_Script_sc_npc_end_behavior
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gml_Script_sc_npc_end_behavior (line -1)
gml_Object_o_player_Step_0