I was threading a cog on and it never seated, when I got to the point that it should have stopped things got kind of mushy. I knew right away what happened so I gave it a few more turns and it stayed mushy and then got easier an easier and then I was certain what I did. The cog is fine, it threaded right on the other side, no problem.

I am 100% sure that the cog was not cross threaded so that was not the cause. Let me tell you about the wheel:

It is a generic Formula hub with Nashbar branding. it is fixed/fixed.
The wheel is about five years old and has seen all kinds of weather.
I ride it pretty much year round here in Minnesota so it gets coated in salt and other corrosive road chemicals.
The non-drive side stays exposed and when I change cogs I usually will flip the wheel so each side should be equally "worn."
I change the cog a couple of times a year as the weather changes and my tires change.
I have never cross-threaded a cog.
I have never had a cog come loose while I am riding.
I have used cheap cogs. Probably some of the cheapest available.
I use a mix of steel and alloy cogs.
I always use a lot of grease and I am pretty good about getting the threads clean.

It is possible that the exposure to chemicals and various cheap cog changes over time just beat the threads up so badly that they just could not take it anymore? (I've inspected the non-stripped side and it is pretty beat up; I am fairly certain that I will blow it out also if I install the cog on it.) Or is it the case that this should never happened and that I must have done something wrong while changing cogs at some point.