Good RID(ance, I mean issuance)
As we know a SID is 12 Bytes long or 96 bits long and is composed of several components, among them the domain identifier and the relative identifier or RID of a particular object. The RID is 30 bits long which means you have approximately 1 billion RIDs. So while you think it is unlikely that you will run out of RIDs, according to http://TechNet.microsoft.com/en-us/library/jj574229.aspx you can encountering this if you have accidentally used scripts or provisioning tools (like FIM) to shoot your self in the foot and create gobs and gobs of users, you let some end-user go out of control creating waaaay too many groups, you increased the RID pool size to be too big, did lots of DC demotion and promotion, cleanups, forest recoveries or invalidated RID pools.