10 Disturbing Doctor Who Implications You Totally Missed

In The Snowmen, the Doctor and Strax try (and fail) to use a memory worm to erase Victorian Clara's memories of their encounter.

Over the years, many fans have noted how interesting it is that the Doctor owns this mind-wiping insect directly after he's just lost Amy and Rory. The Doctor's request for Strax to "get the worm" is evidence of its prior use, and it's clearly something that's been used often, as the Doctor and Strax approach the wiping of Clara's memories as a routine task.

It's possible that the memory worm is actually something from Madame Vastra's collection, used to hide her identity as a lizard woman from the dawn of time. After all, she's later revealed to be aware of Mr Sweet's rare species of red leech. However, the loss of Amy and Rory really hurts the Doctor, and it would be another four regenerations before he became aware of therapy as a viable option for his trauma.

So maybe he has used the memory worm before, to erase the painful memories of the friendships he's just lost, or to clear his mind during his saddest moments. 

The Doctor has been in isolation for quite some time when we find him in The Snowmen, and we know what happens when the Doctor is alone. Maybe the memory worm is the reason he didn't go completely off the rails.