The one where the Doctor loses a companion, gains another and nurses a third...
I find it really poor when Doctor Who introduces a new companion immediately after the previous one has left. It just feels too neat and convenient to be real. The 1970s were the worst culprit, introducing Jo just after Liz left, and Sarah Jane just after Jo left, and Romana just after Leela left. At least in the Hartnell era (and the 1980s, to be fair) there were often overlaps with the characters so that it happened more organically. Steven shared screen time with Ian and Barbara, Dodo with Ben and Polly, and in Horse of Destruction, we're introduced to Katarina before Vicki has left.
It's still a bit convenient though, and it would have been so much better if Donald Cotton had written Katarina into The Myth Makers before episode 4. As a handmaiden of Cassandra, there's plenty of opportunity to have her hanging around, and she could've been given more of a reason to be there rather than the slightly forced presence here.