Once I got ultrasonic coupling from corner to corner of a PCB, coming from an inverter charge pump, as not every switchmode power supply requires inductors.
The design was RF related and the component side height-constrained due to some heatsinking and standard card format requirements.
There was an very low phase noise RF synthesizer, requiring a fairly high (uF's) capacitor for its loop filter, and the only capacitors meeting height constraints were class 2 ceramics, which are piezoelectric and a no-no for this application unless you want to make a very sensitive sismograph (as we discovered later). Area constraints also didn't allow for paralleling too many caps so in the end we just chose the lowest capacitance density class2 ceramic cap that met requirements.
There was an inverter charge pump on the other side of the board which again required ceramic caps due to height constraints. It wasn't strictly required but using it slightly improved (2dB or so) linearity of the ADC preamp and the overall SFDR in consequence for some gain settings.
We power up boards and the output from the synthesizer shows -80 dBc spurs at the switching frequency of the charge pump. Touching the board at some places or installing/removing the heatsink varied the spur level, so we suspected mechanical coupling (the charge pump was separated 20cm from the synth, and shared no power rails with it). We changed the capacitor to something non-piezoelectric on one of the boards and the spurs dropped to -95dBc, but were still there. Turns out that the interference was also sneaking through the decoupling capacitors of the synth's charge pump power rail.
In the end we just removed the charge pump and it was OK.