Perhaps they are minor if there's one or two global variables. Once they're used liberally and there's plenty of them, the resulting bugs can become so elusive it can drive you mad.

A bug can be extremely hard to replicate, because it only arises from a very specific sequence of events that step-by-step flip the globals into an overall invalid state. And it's virtually impossible to determine "whodunnit".

It's a perfect recipe for "the app sometimes crashes on Thursdays" scenario.