The SLD - System Landscape Directory which is a windows service will in some cases, but not all, will for no apparent reason and without warning shut down.
This will cause all users to experience system crashes, delays and general frustration.
At the very least the service will need to be restarted - but all users will have to log out first, then log back in after the restart of the SLD service.
In worst case scenarios the server will need to be rebooted.
There is a hot fix for PL09 which fixes this.
However, the downside to installing the hot-fix is the promise you make to SAP - that as soon as a new patch is available they expect you to upgrade again. As soon as there is a patch beyond the hot-fix patch - the hot-fix patch no longer receives any support.
It is also possible to create a batch file to start and stop this service to simplify the repair prococess.