Blue Linden reports that the Second Life grid is experiencing a presence issue. If you log out, you may not be able to log back in, because - the system hasn't apparently noticed that you're gone yet. The advice is that if you are unable to log back in, wait 30 minutes then contact Customer Support.

The presence system has had a bit of a fraught history, particularly since the day Second Life 1.9 was deployed. Formerly teleportation, sim-border-crossings, instant messages, group invitations and vote-proposals were handled through central servers. Then those servers started to get busier and busier. It was decided, in the interests of scalability, that sims should communicate with each-other in a more direct fashion.

This had a number of effects. Firstly, it made teleportation very much faster -- in many cases less than a few seconds. Secondly, it increased the time taken to cross sim borders from a best-case 0.2 seconds, to a best-case 0.8 seconds. Thirdly, Phoenix Linden noticed that vehicles crossing borders had some problems (we can only imagine that he'd been unaware of it for upwards of two years). And then there was messaging and presence.

Under the new system, to send a message to another resident, the sim you are in has to be able to find out where the recipient is (to 'map' them, in Second Life slang). Your sim forms the message into a bundle, and contacts the destination sim. The message is sent there. This takes some finite amount of time. It's possible that the recipient will leave the sim in that small window.

Once the message has hopped to the destination sim, the sim attempts to ddeliver it to the recipient, if they are still present. If they are, hooray! If they aren't, the sim begins the hunt to see where the user is now. If the message hops more than a few times, or cannot locate the recipient it diverts to an offline message. This can often happen when someone's having a slow teleport because, for a short time, they aren't actually anywhere.

As I understand it, the same message system is used for online/offline notifications, group invitations, group notices and vote-proposals. Sometimes the sim you are standing in fails to communicate correctly with other sims as regards your location, and some of these messages just don't make it. Sometimes the system thinks you are online when you aren't, causing group notices or group invitations to be lost or group IMs not to work right. It's certainly better than it was in the early days of 1.9, but it's still not highly reliable.