Checking the resistance of some cheap Chinese clip leads I bought. The way these are crimped often doesn't make great contact. But even after soldering them, the resistance was still high. The wires were really thin, but it turns out, the resistance was still high considering that. It turns out, the wires, though they look like copper, are made of iron and coated with copper!
Thanks! I tested all my stuff, same result. It's the clip leads and breadboard jumper wires. Some two years and older.
Larger gauge wires were all good, I guess it wouldn't go so unnoticed there.