Well, in seriousness.... (i.e. a non-poo related answer to the question!)
Biggest disappointment was a Fenix TK11 R5 edition. I had the TK11 R2 and TK12 R2. I really liked that series so I eventually ended up getting the R5 edition of the TK11.
What a letdown! They changed the driver so that it was PWM on the low mode!! NONE of the other TK series at that point had PWM. Felt like a bait-and-switch. For the "premium" cost of the light, that was a real downer.
Also, the lens was slightly cloudy. In hindsight I should've sent it back for that reason alone. I think it may have gotten worse over time.
It causes a very funky color change to the beam, it's yellow/purple/yellow in a concentric pattern. I'm not a super-picky white-wall hunter or anything but this was ridiculous.
I thought maybe I'd just take it to work and use it as a beater light that I didn't care if I dropped it off a 100ft catwalk, but even for that purpose it was just too terrible to put up with. The PWM bugs me on top of the tint also.
Major disappointment for a Fenix light; I'd had all good experiences until then, but for me that was the end of my Fenix purchasing (not helped by the fact that I also had a PD31 with flaky mode-shifting; another "premium" R5 light).
The TK11 R5 now sits loaded with cr123a cells, gathering dust as a last-last-last resort, or maybe as an emergency loaner.... to some neighbor I don't like!