In the Common Worship lectionary there are two options for Eastertide. One uses an Old Testament reading, a reading from Acts, and a Gospel reading each week. The other uses an epistle instead of the Old Testament. The point is that Acts is required.
Somehow, I thought we were using the Old Testament readings, and chose hymns accordingly. So this morning we had a lovely reading from Acts, then a letter to Peter or someone, then a gradual hymn which was very much related to the Old Testament reading we hadn't just heard.
Mix-ups do happen. I planned that we would sing "Allelyua, sing to Jesus" as the Communion hymn on Easter Sunday -- what can I say? I like to get as much of that A-word in as I can now we're allowed to say it again. When I had a closer look at it the night before, I realised that the words to verse two are very much more appropriate for Ascension, particularly "Though the cloud from sight received him when the forty days were o'er". Oops! Thankfully I caught that one in time to change it.
I usually choose hymns in advance, about a month at a time, sending the list to the vicar for approval (canon law means the incumbent does have the final say). Occasionally we end up caught in the guessing game of trying to figure out which hymns the choir and congregation will already know, and then find out that no, it's just us who think of some hymn tune as very well-known. I would have expected them to know NUN DANKET ALL for "Jesus, these eyes have never seen", for example, but at rehearsal on Thursday that turned out not to be the case. We used another Common Meter tune instead, there are enough of them about that it wasn't a problem!
The other thing that has happened sometimes is that I've chosen carefully, the vicar has given a thumbs-up to the choices, and then on Sunday morning we both wonder what on earth we were thinking! Sometimes I can follow the thread of my thoughts backward, sometimes not.