(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.
Best Answer
The symptoms are that most of the time everything will be fine, except sometimes it might not be. This can be data-dependent and very hard to reproduce.
Think about what's happening. Some chip suddenly increased its current demand. That caused its immediate power voltage to dip to some level where correct operation is no longer guaranteed. Even if not, the rapid change of the power voltage can cause trouble.
It is very hard to predict what exactly that trouble might be and at what threshold of voltage or derivative of voltage it occurs at. A data line may be temporarily interpreted in the wrong state. A flip-flop may get flipped. You don't know. Whatever happens is also a function of temperature, even uneven heating of the die. Try reproducing that exactly from one test to the next.
So the bottom line is things can get flaky. Maybe. Sometimes.