Mine was: Trying to transition from traditional to hi-rail.
As long as I was staying traditional: These were SUPPOSED to look like toy trains and my rewards/enjoyment level was fine with that center rail, 6464-type proportions, etc. Besides, what's not to love about the smell of ozone and a panache of Postwar? (Mixed in with contemporary traditional-sized trains of the time.)
Then I wanted a bit more realism, so I went from tubular track/switches to GaGraves track/switches. Good move. Nice looking and operating track.
Then I wanted a bit more visual realism so I purchased some scale sized cars via Atlas/Trainman.
Hm... need better running locomotives so I can switch more realistically: Purchased an Atlas SW.
This is getting better.
Then that blasted center rail begin to bother me more and more, as well as the compressed feel of it all...
I chucked it and went back to two rail never to seriously return to 3 rail.
So, my single biggest error in my enjoyment of 3 rail trains? Wanting my presentation to look more realistic.
Andre