Whan that Aprill with his fooles soote...

The Museum of Hoaxes has a section on Medieval Pranks and Tricks, and although some of them are more early modern than medieval, it's nice to find someone has researched this.

According to their page:

The notebook of Thomas Betson*, a fifteenth-century monk at Syon Abbey in Middlesex, records his joke of hiding a beetle inside a hollowed-out apple. When the apple began to mysteriously rock back and forth people believed it to be possessed. Other manuscripts include instructions for more mischievous tricks, such as how to make beds itchy and meat appear wormy.

The Secretum Philosophorum, which was a kind of fourteenth-century guide to trickery, offered a recipe for magically transforming water into wine. The trick was to secretly drop pieces of bread into the water, after first soaking the bread pieces in dark wine and then drying them in the sun.

*Thomas Betson is the author of A Ryght Profytable Treatyse Compendiously Drawen Out Of Many and Dyvers Wrytynges Of Holy Men, as well as the lesser-known A Ryght Profytable Treatyse on Wrytynges Usyng "Y" as Theyr Mayn Vowyl.