Bernice: When it happens, it means that the database server refused to insert your post/move because of a current overload. It's just a feature of database servers and there are two ways, in general, how to fix it:

1) Optimize the application/database code and infrastructure. It is already done and it's called BK 2.0.
2) Optimized code bring one consequence - since the site is faster, it can serve more requests at once. Which, of course, makes the site slower. It's a common problem of all sites with non-sufficient hardware. The remaining solution is to replace the current database server by a new one, faster one.

Please note that we are still running on the same hardware configuration as we started with two years ago. It's a small miracle [thanks to Liquid] that it still runs in kind of acceptable way.