Haven't had a true lemon yet. Closest would be my Marlin 60. It had weird issues with rounds getting jammed up as they fed into the chamber, so badly that the bullet would usually be bent 20-30 degrees out of line with the shell casing. Even when rounds did feed they would usually be "shaved" with a little crescent moon shaped piece of lead removed right at the back of the bullet.
A new feed ramp from Marlin didn't do jack, so I tinkered a bit with the dremel tool. After that (and shoot a couple thousand rounds through it) it doesn't jam anymore, but many rounds still get a light "shave" when they feed. Surprisingly it doesn't seem to hurt the accuracy that much.