I have no memory of being a beginner, in a true sense. I grew up with HO trains as a fixture the same as the TV set, the kitchen table, and mom's 51 Ford flathead. I was probably in 1st grade before I realized that not everyone had HO trains in their basement... or a darkroom... or musical instruments of every kind piled up like firewood at times.
But as I struck out on my own, I certainly made hundreds of mistakes and I still make them. I gather from the subject of this thread, we're talking about a major mistake - one that had long term consequences. We've all paid too much for something, bought things we couldn't use or products that were just disappointing junk. But in terms of decisions to go down a certain path and then later discovering it was the wrong path - well, I fear some of my mistakes have yet to be made. The few layouts I have built have been relatively inexpensive and served their purpose but were never viewed to be a final product. So I have never really attempted to design my dream layout and I'm too old now to be thinking of building 2-3 dream layouts, so I need to get it right the first time and I will solicit help. Fortunately, I know enough people that I know who I can go to for genuine assistance in achieving MY goals, rather than chasing the Kalmbach paradigm of the month.
A couple of paths I went down in my teens turned out to be dead end for me. One of those paths was trying to be sort of a freelance steam modeler, or a steam scratchbuilder. I felt that brass was so expensive I would never be able to afford more than one or two pieces my whole life, so I believed I could build my own. I actually took the MR Cyc drawings of the NYC Niagara and fabricated a really, really bad, lumpy smokebox section from brass before realizing that just wasn't going to happen - not without much better tools and some skills I couldn't just teach myself. I was very much alone then.
One colossal mistake I avoided just by instinct, and often was scorned for not jumping on board, was converting to analog command control before there was DCC. Everything about the existing CC systems waved red flags right and left despite cheerful enthusiasm from the purveyors of these systems. I could see myself spending thousands for a limited, flaky system, being forced to deal with electronics geeks who look down their nose at me, and spending 50% of my train room time cleaning track. I still consider myself both fortunate and perhaps even smart for staying DC until DCC was pretty well established... 1997 to be exact. I've since turned over my system twice, and many of my decoders once, but I don't regret doing it when I did. I am now solidly DCC and would not go back to DC.
OTOH, in 1993 a friend of mine bought an analog CC system... he cut his block wiring and installed the receivers in a number of locos. After about 3 months, he trashed the whole thing and went back to DC. I know, I helped him rewire his blocks.... I just cringed when I found out he had not only bought the bridge, but drank the kool-aid as well.
Recent mistakes? 2008: LOL... how about getting the (2) Atlas C420s I needed only after buying *five* complete locos and still having to order a long hood shell from Atlas.... or (2005) chopping the number boards off a Proto U-boat cab with a single edge razor blade, while holding it tightly with my fat, unprotected left thumb directly opposite the blade?
2002: Running my then-new Walthers Amtrak cars without putting real Kadees on them... turned my back, train uncoupled, locos came around and rear-ended the train and sent brand new Slumbercoach to the floor, cracking end and vestibule.
2007: Attempting to put together an overpriced styrene freight car kit by following the sparse, computer-printed instructions instead of using my own experience and common sense, and trashing the kit completely in the process.
1985: Spending $70 on two NWSL PDT power trucks, thinking they would function as "helpers" by powering passenger cars on my 13-car train to assist the single steam loco, only to find they were poor running, poor pickup, noisy, weak, lousy lead sleds that locked up and ended up being dead weight instead of helpers.
1998: Spending $1800 on brass passenger cars, after working 300 hours in one month and wanting to reward myself with something off the shelf that required no work. Silly me. Cars had junk trucks out of square, dragging shorting brake shoes, coupler mounts in impossible places, and solid diaphragms that couldn't be removed. Manufacturer of said cars called me a liar on a public forum. Unloaded these cars a long time ago.
I'm sure I could come up with lots, lots more....
Andy