When someone is not happy with a trademark application, in most cases an objection will follow during the application procedure. In most countries, the person can also use an opposition procedure. Once the trademark has been registered, an objection can still follow. In fact, there is no time limit for someone to object unless it can be evidenced that acquiescence has taken place. If someone is aware of the use of a trademark application but is inactive (and does not object this trademark), he will not be able to object it anymore after a period of 5 years.
Sounds like a good rule that creates legal certainty. But the burden of proof lies with the defendant. The latter must demonstrate that the other person was aware of the use of the application and remained inactive. And that is not nearly that easy. For example, National Geographic relied on this ground in a recent case. National Geographic pointed out to the EUIPO that 12 years ago the other party had already filed an opposition against the same trademark. No action had been taken ever since. Clear case right? No, National Geographic could not demonstrate that acquiescence took place with regard to use (but only with regard to the application itself). And so the EUIPO rejected this argument.
Fortunately for National Geographic, the case was rather hopeless. Bad faith (!) was invoked an unregistered rights. Both arguments were resolutely rejected.