I found one a few years back when I repaired a linear power supply. This required me to reverse engineer it first because there was no service manual. I buzzed the whole thing out and found out that one of the electrolytic capacitors had both legs connected to ground. They must have shipped thousands of power supplies with that error in it and no one even noticed.
How/what does adding capacitance help with?
It helps to always think of current draw in a compete loop, out the "top" of the capacitor, through your IC, and back into the ground side (this isn't necessarily what's happening physically). Shorter loop means less inductance, shorter traces less resistance.
But I do recall having had inductance issues with high frequency transfers to an atmega.
I have no idea how a capacitor would help. Sounds like it would increase a signal delay (tiny one perhaps).
But definitely interesting, I'll need to learn about this if/when I dive further into electronics.
Capacitance is in many ways the opposite of inductance. If you place capacitors close to the power sink (e.g. the ATMega), the traces will have lower inductance and be “decoupled” from the power supply.