Over and over again the psalms are said or chanted in the Offices. However familiar they are, though, they are not all quite truly memorized. One cannot trust to the automatic pilot in the mind to do its job reliably — and there are times (mercifully rare) when that pilot doesn't work and one has to accept prompt after prompt from other members of the Community.
No matter how often the cycle of the psalms goes round, the words never become ho-hum: a new insight will seep out of a verse which otherwise has slipped by unobtrusively for weeks and one wonders why such a meaning has been so long in coming forward. Other verses are favorites and so satisfying when they fall to one's own side of choir rather than to the other. But what can guarantee a big mistake in recitation is to allow the mind to wander away into secular matters. Maybe the other members in choir aren't counting them, but such mistakes chagrin the maker of them. "Keep awake, then, so that your attention will not be broken into."