Copper is a metal that work hardens and becomes brittle.
I have seen several older bikes where the copper wires have broken where they flex around the headstock.
If you are looking to wire a bike make sure you allow plenty of wire in this area. don't skimp and or use too many Zip ties or spiral wrap.
The loom should just bend gently over several inches, actually it should twist instead of bending.
If you suspect you have a broken wire pull hard on each one at a time, usually if you have a broken copper the insulation will stretch near the break.
Roy

Good advice!  Personally I haven't found problems with mid harness wiring.  But I've had broken strands around locations frequently handled, like around the headlight or signal lights or fuses.

When I built the Led Zed (‘79 KZ1000 LTD), it had some broken wires mid harness.  I asked Roy to help me sort them.  After determining which wires were which, he did the wire stretching trick.  Quickly found the broken spots and patched in some repairs.  Worked well.  Never had any problems afterwards.