I've been doing a lot of Wonderlockes and Wonderwedlockes lately, so I've certainly encountered some interesting trades.
During a Wonderlocke, I got a shiny legit Sylveon (I'm not too big on shinies, but this was the first shiny pokemon I ever got through Wondertrade, and it was a Sylveon to boot, so it was pretty cool)...also got a bred Snivy with the Contrary ability in that same 'locke (it was legit, as the Contrary Serperior event had been going on for at least a week or so in ORAS).
I believe it was the same run where I got a bred Tyrunt with Bite, Ice Fang, Fire Fang, and Thunder Fang. In another run, I got a bred Tyrunt which had Dragon Dance instead of Bite, and the other moves the same.
In a Wonderwedlocke, I got a (legit) Moltres...which I unfortunately had to retrade, because it's genderless.
In that same run, I also got a level 100 shiny Kyurem...obviously hacked, and also genderless. However, I didn't retrade it because I didn't want to send a hacked pokemon back through the Wondertrade (I just felt it was wrong). I know folks are gonna go agro on me for this, but I just released it.
Same run...another hacked shiny, a level 100 Linoone. I knew it was hacked because...come on, a level 100 Linoone? Shiny to boot, so yea...hacked. Nothing wrong with Linoone; I took one through the E4+Champ in my first nuzlocke run of Alpha Sapphire, but nobody would spend time to get one to level 100, shiny or no.
For bad trades...I have been on the receiving end of Magikarp (multiple times)...and I'm sure we've all received our fair share of a bug infestation at some point or another.
Silly trades...I have gotten hilariously named pokemon, but the names are a little inappropriate, so I can't share them here.
If I'm doing a Wonderlocke, I typically try to either name my pokemon "Wonderlocke", or something silly (appropriate-silly; for example, I named a Graveler "Pet Rock", Fenniken "Fox Says", and Marill "Pikablu"...etc). I mostly do that for pokemon people don't really want to see or receive (I once had to trade a male Combee, and named it "I'm Sorry").