When I was little, I only ever trained my started and the legendary (in RSE, anyway), so I ended up at the Elite Four with a Swampert and Kyogre leveled, but nothing else. I knew I needed something else to take on the E4 with, so I caught Victory Road mons to round my team out. I actually ended up winning, but not before I created this atrocity:
Hyper Beam/Blizzard/Thunder/Fire Blast Aggron - I might actually still have this thing somewhere. But yeah, that was...not one of my brightest decisions :P
In addition, restarting with my 80+ Kyogre (or I traded it to my cousin and he did, but same result really).
My pride and joy, the level 100 Rayquaza I painstakingly trained at Sky Tower in Sapphire, somehow got lost along the years to where I am now. Makes me sad, because that thing really was one of my favorites.
Uhhh, what else...
Challenging the Battle Dome/etc with lvl 50 Zapdos/Articuno/Zapdos imported straight from XD, although they actually performed decently well, even without EVs.
Not really a "big" derp, but I used Duking's Plusle in Colosseum, and it actually did really well with the XP boost, being up to 80s towards the end of that game. Lost that one too ):
Lost a level 90-ish Altaria I trained (I was really prone to losing mons :P)
I used to mark mons based on if I would use them or not (one marking for maybe, 2 for little more, 3 for even more, 4 for most, etc), even though I never would use them. It was mostly level based too (so I would see lots of Sky Tower mons with that sorta thing). Cue me now doing breeding and using the markings for IVs, and then it was just sorta funny when I realized why markings would actually be useful.
I re-nicknamed my Swampert "Swamps" because I thought that would make it perform better in contests.
It took me foreeeever to get into/past Sootopolis, mainly because I could never figure out how and kept restarting (using Pokemon Box to store all my mons), so that was interesting.
Was stuck in the Pokemon League on LeafGreen, and I really liked the game, but accidentally left it at a friend's house and never got it back...
Anyway, that's all I can think of for now. But, yeah, small me didn't do too well on Gen III (bar Colosseum and XD), but after a couple of hiccups in Gen IV (never *actually* beating Diamond legit, due to Pokemon Ranch making you keep the same save to access mons and hacking in a full dex so I could port my old mons over at the 5th gym and destroying everything with my l50 Groudon and then never getting around to beating it again after I restarted), I got over it :P