Originally Posted by phidauex

There are indeed bad cogs! The threads on the cog are the only thing coupling the drivetrain to the 'go' of the bike, and if it isn't in good shape, it will cause trouble. If the threaded region isn't deep enough, lockrings won't get enough purchase, and you could slip a cog under rapid decelleration, the force of which can strip threads off your hub. A hub with stripped threads is a useless hub. To replace it, you need to rebuild the wheel, which, unless you do it yourself, probably means new spokes and a new rim as well, basically a whole wheel.

Poorly tapped threads can also chew up your hubs soft aluminum threads, which will make stripping more likely later on.

Lots of people run into these problems here, its not fantasy. Its the reason places like Harris don't sell cheap cogs anymore, they are a false economy. A cog is a simple, but absolutely critical component.

peace,
sam