[bitdiffusion]

Here is one to add to the list - if you delete records and/or entire collections, you won't reclaim the associated disk space automatically. Once the space is allocated, it remains allocated and will be reused when more data is added later. If you want to reclaim the "empty space", you need to run a repairDatabase() which will lock the entire database while it's busy.

[ukd1]

or compact, which you can run on a secondary...good call - missed this one! Will add shortly.