I'm confused, how come Mary references coming over to my place, but the event never actually happens. Also sometimes when i chose to talk to her after the shift, a screen comes up saying "if you're seeing this, an error has occurred" or something... is it even possible to get the Mary ending?
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You've most likely done something out of order. After a quick test I was able to complete a Mary playthough with no issues.
Make sure to check the Relationships tab to ensure you're following the right path. If you want to romance Mary, make sure she comes over at the end of Day 1 + 2 rather than sleeping alone at the hotel.