The elusive invisible right hand bug has been about for a while, for example VWR-26160 and MATBUG-403.
When residents are affected by this bug, it usually only reproduces with certain graphics settings, for example the VWR-26160 case needed basic shaders enabled but atmospheric shaders disabled to reproduce and MATBUG-403 seemed to only affect Mac and needed atmospheric shaders enabled to reproduce it.

Here's another flavour of the bug which hopefully can be reproduced by LL this time.

Steps To Reproduce

• The repro rigged mesh is the Swift hair from Truth, which you can buy here: http://maps.secondlife.com/secondlife/Truth/102/126/34
  See Fig 1 attached for which board to look for.
  (Hair isnt available on the Marketplace).
  The demo will reproduce the bug just fine too but the full version is modify, demo is not.

• I also placed the repro hair in a box here: http://maps.secondlife.com/secondlife/Testylvania%20Sandbox/68/83/22

• Wear the repro hair or get another avatar close by to wear the repro hair.

• In graphics preferences, enable Advanced Lighting Model and disable shadows.
  I am unsure if different systems may need different graphics settings to reproduce this bug - all those that tested so far and reproduced were Windows with Nvidia graphics.

• Observe your avatars right hand and the right hands of other avatars close by at different camera angles.
  The camera angle seems to need to be pointing down on the avatar slightly to reproduce this bug but once angle is correct, you will see the bug at all horizontal camera rotations.

  Observed Behaviour

• When ALM is enabled and shadows are disabled, all nearby avatars right hands will be invisible at certain camera angles.

• The right hands of avatars moving in the scene will often rapidly flicker between visible and invisible.
  See the gifs Fig 2 & Fig 3 attached.

• This bug also reproduces for observers in the scene who are not wearing the repro hair.

• This bug does not reproduce when Shadows are enabled (Sun & moon or Sun, Moon & projectors).

• This bug does not reproduce when ALM is disabled.
  See attached gif Fig 4.

• When the repro hair is removed from the scene the bug no longer reproduces
  See attached gif Fig 5

• Note that when changing the graphics settings or detaching the repro hair, sometimes you need to wiggle camera angle a bit to stop the bug reproducing - not always though.
  Fig 4 & Fig 5 show the effects immediately.

• If you unlink the repro hair and take all prims into inventory, find a camera angle where the bug reproduces and wear each prim one at a time, you will see that the bug only reproduces when wearing one of the prims that is textured with a texture containing some alpha.

• If you edit one of the prims which reproduces the bug, under the texture tab you will see its set to alpha blending.
  If you change alpha blending to alpha masking, the bug no longer reproduces (mask cutoff can be zero or non-zero).
  If you change alpha blending to emissive mask, the bug no longer reproduces.

• If you rez the repro hair on the ground, the bug no longer reproduces - the hair must be worn in the scene to reproduce it.

  Expected Behaviour

  No invisible or flickering right hands.

  Other Information

  Here is the system information from a different system that reproduces the bug with the same graphics settings:

  CPU: Intel(R) Core(TM) i5-3570 CPU @ 3.40GHz (3569.7 MHz)
  Memory: 16342 MB
  OS Version: Microsoft Windows 7 64-bit Service Pack 1 (Build 7601)
  Graphics Card Vendor: NVIDIA Corporation
  Graphics Card: GeForce GTX 650 Ti BOOST/PCIe/SSE2
  
  Windows Graphics Driver Version: 9.18.0013.4752
  OpenGL Version: 4.5.0 NVIDIA 347.52

• Bug also reproduces on Firestorm 4.6.9 (42974) Dec 12 2014 09:47:51 (Firestorm-Release) with Havok support and Firestorm 4.7.0 builds (Merged up to LL default release codebase).

• Bug does not reproduce on Singularity Viewer (64 bit) 1.8.6 (6157) Feb 26 2015 19:38:50 (Singularity) with any combination of graphics settings.

Attachments

• Fig 1 - Swift hair.png 2015-03-02 04:16:59
• Fig 2.gif 2015-03-02 04:31:07
• Fig 3.gif 2015-03-02 04:35:26
• Fig 4 - Graphics Settings.gif 2015-03-02 04:44:18
• Fig 5 - Detaching hair.gif 2015-03-02 04:44:51

{
  'Business Unit': ['Platform'],
  'Severity': 'Unset',
  'System': 'SL Viewer',
  'Target Viewer Version': 'viewer-development',
  'What just happened?': '.',
  'What were you doing when it happened?': 'Filling in...',
  'What were you expecting to happen instead?': '.',
}