He Misheard
During the weeks before Amy's wedding, she was terribly anxious about making some mistakes at the ceremony.
The minister reassured her several times, pointing out that the service was not difficult and she will do just fine.
"All you have to remember," he said, "is that when you enter the church you walk up the AISLE. The groom and best man will be waiting before the ALTAR. Then I shall request the congregation to sing a HYMN... then we shall get on with the ceremony. All you have to remember is the order in which those things happen and you can't go wrong."
The happy day finally arrived, and the bridegroom waited nervously for his bride to appear.
When she arrived and stood alongside him, he heard her quietly repeating to herself, "Aisle, altar, hymn, aisle, altar, hymn." Or, as it sounded to him, "I'll alter him!"