This page is a compilation of information related to inventory loss, which has a number of possible causes which need to be distinguished in order to determine what's going on. For example, see SVC-242 for a now-fixed bug with a solid repro (series of steps needed to make the problem happen).
Also be sure to see SVC-114 on the Issue Tracker, which is a "Meta-Issue" of various inventory loss reports we've keeping an eye on. We unfortunately can't do much without details, so we emphasize that the Issue Tracker should be used to report specific bugs. If you're looking for individual account help, please send a ticket through our Support Portal.
Sometimes, there's perceived inventory loss- "false positives" involving inventory simply not showing up (because you have to wait awhile) or need to clear your cache to refresh your items. Keep reading on for more info about things you can try to rule out actual inventory loss.
We (Lindens) are continuing to make progress on inventory loss issues: this includes initiatives such as better metrics (including quantifying how many verified losses are happening), improving the backend architecture of Second Life so that inventory loss is less likely to happen, and we plan to communicate further details on our Official Linden Blog.