The Fifth Doctor faced off twice against the same enemy, the psychic snake monster/parasite called the Mara, in "Kinda" (season 19) and "Snakedance" (season 20). While both stories are pretty well done (and, in many respects, "Snakedance" seems better made, especially with regard to the special effects) there were some disappointing changes made between the stories, and I have always wondered why.
In "Kinda," the Mara is defeated by surrounding it with mirrors, because it cannot bear to look at its own evil reflection. (Pardon the extremely dubious snake effects.)
In "Snakedance," the Mara, which is grappling with Tegan for control of its body, actually uses its reflection (featuring a snake skull in place of a head, with the motif of the skull being another new aspect in the story) to freak Tegan out.
Tegan (quite correctly, in my opinion) points out the continuity error, but the Mara just waves her objection away without any real explanation.
Another jarring change was in the way the Mara was passed from person to person. In "Kinda," there was just one Mara (or, at least, just one that ever got loose; Tegan does see some other people with what might be Mara tattoos while she is psychically projecting into the "dark places of the inside"). Most of the time, the Mara manifests itself as a serpent tattoo on the forearm of the person it is possessing. When it wants to take over someone else, the two hosts like hands; then snake comes to life and crawls over to its new host, where it returns to tattoo form. In "Snakedance," the Mara does not move from person to person. Instead it seems to spread like an infection; once a person it possessed, they can spread the possession to others without themself being free.
The same writer, Christopher Bailey, was credited for both serials. I am wondering whether there is any documentation from Bailey or anyone else associated with the show discussing why the nature of the Mara villain was changed. The first story has a clear sequel hook at the end, suggesting that perhaps the Mara is not really entirely gone from Tegan's mind; and that was exactly how things played out in "Snakedance." So there was clearly at least a vague notion that the "Kinda" could have a sequel. Why was it decided to be necessary to make so many changes of approach when that sequel was made?