It definitely can be. The important thing to understand about voltage is that voltage is always relative - it only makes sense to talk about a voltage difference between two points. Within a given circuit when we say “this output is at 5V” this is just shorthand for “the difference in electrical potential between this output and ground is five volts”, where ‘ground’ is some designated reference point in the circuit.
But whenever you have two different pieces of equipment that don’t have a physical connection between their ground points, it’s possible (even likely) that there’s some potential difference between what one device calls ground and what the other calls ground. That potential difference could be almost 0 volts, a few volts (causing patching confusion), or 5,000 volts (causing damage). If there’s a large voltage between grounds for two pieces of equipment then it’s possible that lot of current could flow when you connect them, if there’s only a small difference they could behave normally. Without electrical contact between the ground points of your different devices, you just don’t know, and the potential difference between grounds could vary over time.
Devices may share a ground this way if they use the earth ground provided by the power strip / wall socket. For DC power supplies (e.g. wall warts or switcher bricks) this may not be the case and two DC supplies plugged in to the same strip may not have the same DC ground on their output (barrel connector or what have you). Using a battery, the battery’s ground terminal is also unrelated to ground from your wall-plugged power supplies.
