Okay, if you don't like this thread, don't blame me, I was pushed into starting it by She Who Shall Not Be Named. :p
The idea: list all the plot holes in all the Sherlock episodes.
Then, try to fill the holes.
For example:
How did Mary get into CAM's office in HLV without being buzzed up in the elevator by Janine?
Possible solution: through the window, according to Moftiss, but feel free to suggest other, more realistic solutions.
It'd be nice to list the plot holes in episode order, but as I am one of those people who rarely notices a plot hole until I've fallen through it, I will have to leave that bit to someone else.
Okay, go find plot holes! First response wins a no prize.