Heater voltage.

[Nick]

Also piezoelectric!
A problem with many ceramic caps.

Charge up a 50 year old 1nF 5 kV cap and leave connected for 20 minutes Short it, isolate it. Come back 20 minutes later and measure the voltage. It will be 100s of volts. The 5kV deformed it. So later it "recovers" and voltage rises as it does. Even more "interesting" with 10 kV or 20 kV parts all in parallel on a soak testing tray. The solution is NOT to short the tray, but put a bleed resistor.

You can guess how I discovered  this 50 years ago.

When I was much younger in South London, we used to spend a bit of time in the transmitter rooms at Crystal Palace (under the hill). All the BIG capacitors there (mainly PIO ISTR), when not in circuit, had their terminals shorted otherwise the standing RF used to charge them up to lethal levels. We had a wooden "de-bollocking stick" which was used to ground or short out anything you were working on for safety, else death would be certain, quick but painful.

[DrStrangelove]

Apparently 100uF is available in 0805.

I read somewhere recently about a 100uF 2.5V in something even smaller.

All of which are impressively microphonic.

[ppppenguin]

When I was much younger in South London, we used to spend a bit of time in the transmitter rooms at Crystal Palace (under the hill). All the BIG capacitors there (mainly PIO ISTR), when not in circuit, had their terminals shorted otherwise the standing RF used to charge them up to lethal levels. We had a wooden "de-bollocking stick" which was used to ground or short out anything you were working on for safety, else death would be certain, quick but painful.

We have some big ex-transmitter capacitors at the Broadcast Engineering Museum. We make sure they are safely shorted out. We're not attempting to run our 40kW transmitter

We have an earth wand (ex Crystal Palace as it happens) but it's not yet connected.