Short version: Set bonuses for Gothic and Phaseshift appear to be behaving oddly. No intentional unusual behavior, prevents further gameplay, occurred on L83 of Ao100.
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I picked up a set of Gothic and Phaseshift equipment.
As I recall from previous versions, putting on set equipment would instantly [or close enough] trigger their set bonuses. This time, I blew out a bridge over a lava river and decided to phaseshift back over it... and never levitated. Ran around a bit, nothing happened. Okay, well, since my boots were modded, maybe I accidentally destroyed the levitation mechanism or my mods were too heavy. *walks over lava*
Then, I ended up against three arch-viles and a Revenant due to sloppy BFG9k firing, so I decided to go fortress-mode. Nope, that didn't trigger either so back to popping medkits.
A few floors later, I notice I find an extra power mod and decide to upgrade one of the spare red armors I've been hauling around for a while. I swap it with my Gothic and..."You feel like a fortress!" And no amount of further armor swapping will get rid of that. Just to be sure, I even dumped all my stuff on the ground. *sighs at L83*