(This question occurred to me as a result of a different question here.)
I am usually fastidious about using decoupling capacitors near all power pins on ICs, large and small, analog or digital. I also use power and ground planes in PCB designs when possible. Generally, I try to use "good practice" in order to obtain reliable robust design. And, as far as I can tell, I've been successful.
The question is, what are the indicators of inadequate decoupling. Suppose I decided to not include the bypass caps at the power pins of a microcontroller or CAN transceiver, or something else.
There are some obvious indicators like the microcontroller spontaneously resetting, but there must be more subtle problems that I might not even see, or might not attribute to inadequate decoupling.