Well,
If you switch off external power with higher loads (fuel pumps on) the noise on the contactor is louder. Also you see the monitors (PFD, ND, ECAM etc) to momentarily go off and on (like a white flash) and you can feel it hurts the plane. I dont know if thats a placebo on me, but I have encountered that many times. They dont happen all the time. We fly mixed fleet of 15 yrs old 320s but also NEO.
Regarding the ECAMs that sometimes accompany the above, I have seen many in my career that occurs when removing external power on high loads.
Examples that I have seen: SEC FAULTS(1,2 or 3 and sometimes two of them simultaneously), ELAC FAULTS, LG SYS DISAGREE, SMOKE LAV DET FAULT, FLAP SYS 1 FAULT and for sure some more, I cant remember all of them.
Its very rare, but happens. Calling the maintenance for clarifications and help, usually they guide you through MCDU system test pages. The tests pass almost all the time and the only solution is to completely power off the aircraft for 2-3 minutes and then power on again. Which is an issue if you have a fully boarded plane.
Then the problems disappear like magic. Its a known issue, happens rarely, but maintenance knows about it and its due to electrical switching. However doing this trick with the battery, saves the day and you feel the transit is a lot smoother.
By the way, not exactly relevant, but this morning the APU didnt pass the fire test during the preliminary, there was no response at all when pushing the test button. Maintenance advised to pull all the CBs behind and then push them on again. Problem solved.