Connecting the current terminals of a multimeter in parallel to a power source is a serious mistake. It can result in minor damage (if the fuse in the multimeter is working and of proper rating). In the worst case, it can result in a dangerous arc flash which can result in injury or death.
Hence, always ensure that the probes are in the voltage terminals when the multimeter is being connected in parallel. Some multimeter have a blanking mechanism which closes the current probes when the selector switch it moved to a voltage measurement position.
Always check the probes and the terminals when you measure the voltage.