Depends on the car, some cars have other things connected to ground or have the engine/chassis ground connected to the same cable as the negative battery cable. Most electrical systems are pretty much identical and have all of the same things connected together but can have big variations in where everything is hooked together. So you might have inadvertently removed a ground connection that the electrical system needs.

You can connect a ground from wherever the ground cable was connected to directly to the chassis (or engine, depending on how the stock system was hooked up) and that should solve the problem. Extra grounds are never an issue, unless you start putting grounds in strange spots which can sometimes introduce high frequency noise.

Really though, you should have seen something extra other than just the negative battery cable if that is the problem, like a smaller cable attached to the battery ground, etc. It's hard to explain.