Some circuits requires more than 1 power source for voltage comparison or whatever reason and the circuit requires that the power sources shared the common ground. Most of the time it works perfectly but are there limitations to this ?
Is the common ground trustworthy ?
Can some mA go from one power source to another ? is it problematic/safe ?
You need a point where the two power supplies connect, and most of the time this is the ground of both supplies. If you don't connect them they will "float" with respect to each other, meaning that the ground (or any other level) of the second supply doesn't have a defined level with respect to the first.
If they're not connected by a common ground no current will flow from the output of one circuit to the input of the second, since there will be no closed loop where the current can flow back.