Upstream incorrectly handles attachments for incoming hypergrid visitors.
You teleport to a hypergrid region, only to find out they broke your AO, RLV relay, open collar, dance HUD, naughty bits, eyeblinking, jaw movement (typing/voicing) etc. In addition, attachments have not been set to the group you have active on this foreign grid, causing llSameGroup() to fail in scripts.
I fixed this in OpenSim/lickx, as long as the destination sim runs our fork. It is important to know that this implementation is NOT the same as the insecure 'TrustBinaries = true' setting, which most sims shouldn't use.
When HG teleporting to a sim running OpenSim/lickx, your avatar will load progressively, as each attachment has completed fetching assets it will resume its scripts (on the fly recompiled) with all state intact, and become visible to people nearby. HUDs are loaded last, as they aren't visible to onlookers anyway and can contain a large amount of assets such as AO animations.
Depending on the amount of worn assets (on or inside the attachments), the speed of your home grid's asset server, and the speed of the destination grid's asset server, this can take between a few seconds (healthy grids) to a few minutes in worst case (for example when hypergridding to a sim on OsGrid).
So far only this fork has this bug fixed. That 'just' leaves 99.999999% of the rest of the hypergrid broken. But at least hypergrid visitors to our sims will have a pleasant experience.