By default, if you reference the same file twice, on file open Maya copies the existing nodes instead of re-reading them from disk. Occasionally, using this built-in multiple reference optimization feature of file referencing can cause errors. The MAYA_FORCE_REF_READ environment variable turns off the file referencing optimization and forces reference files to be explicitly read in all cases. This fixes Maya's behavior in some situations that would otherwise be evaluated incorrectly.
For more information, see General variables.