Last night I was installing a ceiling fan in our relatively new house (built in 2005). It was pre-wired for ceiling fans, so this should have been an easy installation. However, after twisting the wire nut onto two of the conductors, I noticed that the insulation on the THHN jacket had literally split and exposed the bare copper wire!

Admittedly, I did crank those wire nuts on tightly, but I have never had a problem with insulation splitting like this before. I ended up having to cut it back about two inches. Has anyone else had this occur before?

I have seen a lot of the teflon jacket splitting off but the thermoplastic insulation always seems OK.

Must have been Chinese made wire or something!

Just wondering, did you remove the NM outer jacket with a utility razor knife or maybe use those Klein NM cable stripping pliers?
I've often seen the type of problem you described when these tools are used.

Hard to say what kind of tool was used by the original installers, this is a tract home, but your suspicions sound quite plausible.