Heater voltage.

[ppppenguin]

It's worth adding that these problems don't affect low value ceramics, sometimes known as NP0 or C0G, which are accurate and stable. As you go over 1000pF you need to use dielectrics that have worse properties ushc as X7R. Not too bad up to 100nF, pretty horrible at 10uF. I don't know if z5U is used all the way up to the very high values or if there's something even nastier.

They have their place - a lot of decoupling can be done very well with nasty dielectric ceramics but be very wary of using them for many applications.