Whenever I ask questions like this, I try to make the subject something that would make for an easy search result so that it might help people in the future. With that in mind, it's also important to follow up. Aaaand to be honest.

That being said...I was wrong! >:( lol

Switch -> PoE injector -> MX2Wire Transmitter -> (~1000' of 2 conductor) -> MX2Wire Receiver -> Mobotix T25 Door station

^^ That's the setup. During regular operation, the T25 at the far end is powered by the PoE injector, as you can probably assume. When this door station went down, I assumed it was a faulty T25, but on the bench (directly into a PoE source), it tested out fine. OK, so now it looks like a bad MX2Wire set. Got a new set and the T25 still didn't work (this is when I thought the wire could be the issue). I ended up moving the camera from the closer location to this location, and that worked. Also, when I bench tested the 'bad' MX2Wire set with the camera from the closer location, it 'kinda' worked - I got screen shots, but couldn't get it to display a video stream.

So what I have is some parts that aren't fully broken, but they just can't do what they used to do. I never considered electronics to be at all like a mechanical device, which wears out and degrades over time, but it sure looks like that's what has happened here. Along the lines of what you were just discussing, I've seen power supplied do weird things, which I suppose might be similar.