I've found I've been making the following mistake at Prime:
Correctly, I finish the invariable little chapter Regi sæculorum (I Tim. i, 17) by saying Deo gratias, thus:
Regi sæculorum immortali et invisibili, soli Deo honor et gloria in sæcula sæculorum. Amen.R/. Deo gratias.
(Unusually, this sentence of Scripture includes the word Amen, and then one must respond, as to all Scripture, by saying "Thanks be to God" for this word of the Lord.)
Incorrectly, by a faulty subconscious analogy, when I soon after recite the prayer Sancta Maria, I make to end it also with Deo gratias, even though it should read:
Sancta Maria et omnes Sancti intercedant pro nobis ad Dominum, ut nos mereamur ab eo adjuvari et salvari, qui vivit et regnat in sæcula sæculorum.R/. Amen.
The ...in sæcula sæculorum. Amen. Deo gratias. of the former makes me tend to add a superfluous Deo gratias to the latter.
Very minor, I know, but a good example of why one ought pay careful attention to what one is praying.