One side benefit of standardization is that parts that should not ever fit together sometimes do. Most readily available bicycles conform to ISO standards, with fixed cogs, single- and multispeed freewheels, and left hand bottom bracket cups sharing the same 1.37/1.375 in x 24 tpi threading. This means that fixed cogs and old-school bottom bracket lockrings thread onto freewheel hub threads, making for a tempting combination for the cash strapped who want to go fixed. Older, multispeed freewheel hubs and complete wheels are a dime a dozen, and they are easily respaced for a single speed drivetrain. It’s not a far leap to notice that a fixed cog and bottom bracket lockring fit, and at first glance even go together snug. There you have a suicide hub, and mechanically a terrible idea.
Traditional track hubs feature right hand cog threading and a stepped, smaller diameter left hand thread lockring (1.29 in x 29 tpi ISO, 1.32 in x 24 tpi Italian). This configuration ensures that, installed properly, the fixed cog does not come loose with the force of backpedaling, as friction between the faces of the cog and lockring forces the lockring tighter as the cog unthreads. In a suicide setup with a right hand thread bottom bracket lockring and standard freewheel threads, backpedaling forces both the cog and lockring loose. Some swear it works, and I’d be a liar if I didn’t admit my own experiments in the realm, but a fixed cog without a reverse thread lockring is just a bad idea and absolutely not recommended. Even with thread locking compound or epoxy holding it in place, you’re just asking for it to unthread just when you need it to hold tight.
If you’re on the market for used bikes via swaps or Craigslist for yourself or friends, you should be on the lookout for suicide fixed conversions of older road bikes. They’re fairly common amongst those flipping bikes for a profit, and are definitely a mechanical risk that one should not take lightly. Respaced single speed conversions are completely legit, and hubs with proper stepped fixed cog and lockring threading are common enough that there is no reason to settle for anything else if you’re going fixed. Even if you are so far successfully already running a suicide setup you should stop and switch—the retail and used market is saturated enough that the safe and correct hub is only a few dollars away.