In a recent support case, a customer reported he was getting an error that looked like this:

' INFO : Rendering frame 123 (62.5% done)
' FATAL : DB   0.14 fatal  041052: accessing unknown tag 0x41d
' FATAL : mental ray has encountered a fatal error condition.
' It has been disabled for the rest of this XSI session.
' Please save your work and and contact Softimage support (support@softimage.com).

Sometimes “accessing unknown tag” errors can happen when you run out of memory. Other times, “accessing unknown tag” errors can indicate that something is missing in the scene database created by mental ray. Everything in the scene database has a tag, including shaders and light profiles. Shaders use the tags to reference objects, so if some input to a shader is missing, you would get the “unknown tag” error.

In this particular case, the customer was able to resolve the problem by removing all unused materials and image clips from the scene.