Depends how the fuel lines to the pumps are connected. If in parallel then I can understand the scenario described by Mike. But if in series then I'd be concerned if that happened whilst the engine's running.
The vapour return line is connected at the highest point on the system and returns any vapour bubbles to the fuel tank via an orifice which prevents undue reduction in pressure. Many Rotax powered aircraft are running mogas which has a higher vapour pressure than avgas and therefore higher risk of vapour bubbles forming which could prevent fuel flow, hence the insistence on having a return line.
When the engine is stopped and the electric pump switched off the fuel pressure should drop to zero after about 10 seconds. If it doesn't then that's an indication that the return line might be blocked (usually at the orifice) and needs to to be checked.