As I posted in an article on December 20, Linden Lab is no longer offering Viewer 1.23, the download is gone from the official download page. Thanks to Tateru Nino for spotting this notification from Oz Linden (I am unable to find where Oz posted it). LL is deploying changes to the inventory backend that will need a patch in all viewers and they have made the decision not to include this patch in 1.23.
If you are using Viewer 1.2.3 you will have serious inventory problems beginning some time in the next few weeks when this change is deployed into the asset servers.
If you use a Third Party Viewer (including V1 based ones) you can expect an update very soon with the necessary patch.
My suggestion to those using V1 viewers is to bite the bullet and switch to a V2/3 viewer, this is only the first in a series of updates that will eventually break V1 for good.
I wish there were better alternatives for those of you who struggle in SL with V2/3 viewers 😦
Update: Nalates has a very good explanation of this at her blog.