The Swarm
In case you skip it...
In this episode, The Doctor’s program runs into some technical issues that ultimately result in him being reinitialized and his entire memory since his original activation being lost.

You’d think this would have a lasting impact, but it really doesn’t. The only reason I even mention it is so that you won’t be confused when, a few episodes later, The Doctor claims that he is still in the process of recovering his memory files.