Can a pure devotee fall down?
One who is under the protection of the Lord’s internal potency cannot fall down. Those whose hearts are completely pure will not fall down. But even at bhava-bhakti there are traces of anarthas, which can lead to falldown. Anarthas are not completely destroyed until prema. Bilvamangala Thakura had reached bhava-bhakti in his previous life, but he temporarily fell from that platform.
(LFD, Vol2, Becoming Free from Sin, Pg 43, 2nd paragraph)