This article is from the FAQ, by with numerous contributions by others.
[corrected in r4.0.1]
Some of the internal restructuring of the compiler that has been done for
version 5.2 has caused a new bug to appear. The symptom of the bug is that
.bet sometimes gets appended to the file name reported, when e.g. an attempt
to write a file fails because of permission problems.
An example is
You are attempting to translate the file
You do not have permission for doing this!
in case the file /users/smith/beta/private/sun4s/foobody..db, or the
directory it resides in, is write protected.
There is currently no workaround for this bug, but it is usually obvious
what file name should have been reported.