That's how many different versions of different viewers were reported to log into SL last week, according to Oz Linden at today's third party viewer meeting. You can see the recording on YouTube. The discussion starts about nine minutes in.
One thousand six hundred sixty five.
Think about that for a moment.
That's beyond anyone's ability to test thoroughly. The combinations quickly grow well beyond insane to downright impossible.
Even individual TPV teams can't test their own viewers thoroughly. Out of that 1665, 50 are different versions of Phoenix, 151 of Singularity, and 262 of Firestorm. Many of the latter two are probably developers and self-compilers, but still...LL has real money to pay for resources to sit and test viewers. Those resources aren't endless, and the time certainly isn't. If LL can't, a team of volunteers like the Firestorm team damned sure can't!...never mind even smaller teams like Singularity, or individual developers like Henri.
And that doesn't even begin to take into account viewers for which the maintainers have long since given up, or left SL, or simply disowned the code.
This came up in the context of a serious bug in the server-side baking code. The bug causes people using Phoenix or 1.23 who teleport into a server-side baking region to be prompted to save changes to their avatar - and that save corrupts modifiable skin, eyes, or system clothing. Permanently. Unrepairably. Gone. Poof.
Oz's point was that they'll try very hard to fix the bug - but they won't know for sure they've fixed it, simply because it's impossible to test all the cases. All they can do is hope, and if you are using an old viewer that doesn't support SSB, you're going to be out of luck.
My point is much simpler than this.
UPGRADE YOUR FUCKING VIEWER!
Really, people. There is NO EXCUSE any more for running older viewers that don't support the way SL works now. By now, you can even get second-hand machines for not a lot of money that will run Firestorm just fine. You're going to have to bite the bullet.
We put in a metric crapload of work to make Firestorm something that people of all abilities - even those firmly wedded to Phoenix - can use and use effectively. If you don't like the result, fine. Get Viewer 3. Get Singularity. Get Exodus. Get CoolVL. Get something, anything that supports SSB, but do it now!
If you don't, don't come crying to me when your inventory gets ravaged by some bug that nobody could reasonably test for.