For me, the worst offender was a Savage Mk II G, 22LR.
Savage makes an accuracy guarantee of 1.5" at 100 yds with their sporter weight 22LR barrels. Mine wouldn't do but about that at 50 and the bore looked like corrugated pipe. I sent it back to Savage, who replaced the barrel with another that only looked like corrugated pipe in the last 1/4" . It was better, but not as accurate as promised, so I pillared and bedded the receiver. It's now a consistent 2 MOA gun with quality ammo but still not as advertised.
I've given it to my nephew and replaced it with a CZ 455 Varmint that will shoot circles around it with $6.25/box SK Standard +.
The other one was a H&R Ultra Handi Rifle in .223. It was a solid gun, but accuracy was inconsistent and I had to use Burris Signature Z rings with .025" of offset inserts to get the damned thing on paper. Sold this rifle a few years ago and don't miss it.