The incompleteness of the Tales led several medieval authors to write additions and supplements to the tales to make them more complete. Some of the oldest existing manuscripts of the tales include new or modified tales, showing that even early on, such additions were being created. These emendations included various expansions of the Cook's Tale, which Chaucer never finished, The Plowman's Tale, The Tale of Gamelyn, the Siege of Thebes, and the Tale of Beryn.
The Tale of Beryn, written by an anonymous author in the 15th century, is preceded by a lengthy prologue in which the pilgrims arrive at Canterbury and their activities there are described. While the rest of the pilgrims disperse throughout the town, the Pardoner seeks the affections of Kate the barmaid, but faces problems dealing with the man in her life and the innkeeper Harry Bailey. As the pilgrims turn back home, the Merchant restarts the storytelling with Tale of Beryn. In this tale, a young man named Beryn travels from Rome to Egypt to seek his fortune only to be cheated by other businessmen there. He is then aided by a local man in getting his revenge. The tale comes from the French tale Bérinus and exists in a single early manuscript of the tales, although it was printed along with the tales in a 1721 edition by John Urry.
John Lydgate wrote The Siege of Thebes in about 1420. Like the Tale of Beryn, it is preceded by a prologue in which the pilgrims arrive in Canterbury. Lydgate places himself among the pilgrims as one of them and describes how he was a part of Chaucer's trip and heard the stories. He characterises himself as a monk and tells a long story about the history of Thebes before the events of the Knight's Tale. John Lydgate's tale was popular early on and exists in old manuscripts both on its own and as part of the tales. It was first printed as early as 1561 by John Stow and several editions for centuries after followed suit.
There are actually two versions of The Plowman's Tale, both of which are influenced by the story Piers Plowman, a work written during Chaucer's lifetime. Chaucer describes a Plowman in the General Prologue of his tales, but never gives him his own tale. One tale, written by Thomas Occleve, describes the miracle of the Virgin and the Sleeveless Garment. Another tale features a pelican and a griffin debating church corruption, with the pelican taking a position of protest akin to John Wycliffe's ideas.
The Tale of Gamelyn was included in an early manuscript version of the tales, Harley 7334, which is notorious for being one of the lower-quality early manuscripts in terms of editor error and alteration. It is now widely rejected by scholars as an authentic Chaucerian tale, although some scholars think he may have intended to rewrite the story as a tale for the Yeoman. Dates for its authorship vary from 1340 to 1370.