I cleaned up my schematic with several global labes. Unfortunately, I had a typo in one of these labels so that one global label was connected to nothing. When I started the simulation everything ran as usual, without any errors message. The error popped up much later which made it very hard to find. Is there a chance to get a warning if a global is not connected to any other label?
ngspice does not check for the validity of a circuit, except for those errors that would prevent it from creating a circuit matrix which the ngspice solver can handle.
I even use (unconnected or connected) global labels for naming nodes in preparation for simulation. The advantage is that KiCad uses these label names exactly as they are given as node names for ngspice. Local labels will get a ‘/’ prepended, which sometimes may cause some trouble in ngspice.
So you should have a look at a KiCad tool for checking your circuit before simulation. What about an ERC (electrical rule check)?