If a net with two different labels is created in KiCad, then the nets are merged silently, and KiCad just picks one of the labels (I think alphabetically) as the net name.
So if you create a short between “+5V” and “GND” then these just get merged. There may be an ERC error, especially if both nets have a power output (an important reason for the “PWR_FLAG” symbol).
The location of such a short is often hard to spot. A common mistake is to draw a wire right through an schematic symbol (for example a decoupling capacitor).
One way to spot these is to looking for unexpected junction dots on the pins of the symbols. See screenshot below.
This short is caught by ERC because both nets have a PWR_FLAG, and thus a power output pin. If one of these PWR_FLAGs is missing, then ERC does not catch it.