Both sentences are acceptable:
I took the wrong bus.- though one is more apt to hear "... the wrong bus."
I took a wrong bus.
In both sentences, the phrase wrong bus here means wrong bus route, that is, a bus that took one to an unintended destination.
In the case of the following sentences, however, one could infer from the cause and effect implied that the bus ride was not up to expectations, i.e., that the conditions of the ride itself were substandard:
I took a wrong bus. (???) The cushion was bad, and it was not punctual at all.Thus, the phrase wrong bus -- which, again, means wrong bus route -- doesn't make any sense here. Better:
The bus ride was _________ (here, take your pick or supply your own: inconvenient, horrible, taxing, a trial, an endurance test, etc.). The cushion was ... etc.