Allentown references two traumatic memories

People have already pointed out that Dr. Mauer’s “I said, ‘I love you’” echoes Gemma’s last words to Mark. I just rewatched the episode and noticed that during Gemma’s miscarriage - possibly the worst moment of her life - the camera focuses on the grout in the bathtub. That seems like the exact sort of detail that would lodge itself in your subconscious during a traumatic experience.

In Allentown, Gemma is writing a thank-you note for an electric (de-)grouter. “We look forward to the many happy memories it will provide,” she says.

My guess is that before creating the rooms, they check their subjects for emotional reactions to various neutral words (maybe that’s part of their “personality testing” bullshit). Then they incorporate those words into the room’s activity to monitor bleed-through.