Bhai Gurdas was a pooran brahmgyani, way ahead than present day babaas.
He made a mistake and wrote:
If Guru tests His sikh, sikh shud not waver....
Guru Arjan dev ji told Bhai gurdas to correct his line, Bhai Gurdas corrected it by writing:
If Guru tests His sikh, what can the sikh do?