I used to believe that the phrase "Lord, have mercy" was "Lord, how mercy". At the time, I was familiar with the word "mercy" but had no idea what it meant. From the context of what I thought was "Lord, how mercy", I assumed that "mercy" must be an adjective, indicating the presence of lots of "merce" or "merces", whatever merse or merces were. I So I grew to think that "merce" or "merces" denoted a condition of difficult or dreaded circumstances. After all that was the kind of conditions generally being experienced or anticipated by anyone who said (so I thought) "Lord, how mercy!".
A childhood belief about speaking

others believed things like this
- I used to have to say the Lord's prayer everyday, but I would wonder where my daily bread was.— caledonia
- When I was learning the Lord's Prayer, I used to say, "Our Father, who does art in heaven...."— Karen
I used to believe that the Lord's Prayer instructed us to lead a snot into temptation. — Susan- "Hail Mary, full of grapes, the lord is a tree"— reenie
- I used to htink that the Lord's prayer went "and lead us not into temptation, but deliver us some email..."— Dyth
- I used to believe that in the Christian Prayer of the Faithful, we were responding, "Lord, harrow prayer."— Smelleanor