In no particular order

- For reasons that make no sense, the summary page can occasionally get
  negative numbers in the results. That's what sanity.php is for, but this
  script causes the tracker to become unresponsive while working. Use
  sanity.php?nolock=y to avoid.
- Speed tracking is rough at best. When starting or after long periods of
  inactivity, it might take up to half an hour to become accurate. It will
  most likely be understated most of the time.