Ohmic probe aborts immediately

Switches wear out over time and require less force to actuate, which can make a slight difference, bearings wear down a bit, screws loosen slightly, etc.

That's what I mean by mechanical changes.

By mechanical shifting I mean like a leadscrew no longer being in exactly the same place after years of operation due to slight movement of stuff over time.

SIDE NOTE:
My grandparents recently bought a house that was built in 1974 and were told it would not shift or settle because it was built long ago and has had time to settle for 50 years.
But vibrations caused by renovations caused the walls in the basement to crack a bit visually (structurally, it's all fine).

At long last, here it is:

Ouch. I swear I knew that before. Thanks a lot for your reply.

For anyone else that stumbles on this, here is what I added to my personal operations manual:

Q: Ohmic sensing is triggered early
A: This is most often caused by slag or water getting into the shield. 

With the torch shield away from anything conductive, tap the ohmic test button. If the Ohmic light comes on then the ohmic sense circuit is shorted. If you are doing test cuts on scrap, tap the Torch Pulse button to blow out any water in the shield. If that doesn't work, turn off plasma cutter and blow the tip with compressed air. If this does not work, remove shield and clean out any debris. Look for slag that is stuck to the electrode and/or shield cup. 

In the case of slag, consider increasing pierce height as this almost always happens during pierce. In the case of water, add G0 to your program to rapidly move the torch away from water being sprayed by the a torch during postflow.