"Error: bad value" problem

Dear R-help,

Like several other subscribers, I have recently encountered a problem
whereby R will execute code apparently correctly and without error,
but any subsequent command will yield "Error: bad value" so that R
has to be killed and restarted. We have checked this out with a few
different operating systems (Windows XP/Vista and Linux) and with
different versions of R. We have established the following:

1.	The error occurs with versions of R from 2.5.0 onwards on all the
OSs we have tried, but not with earlier versions of R.
2.	The error is not reproducable between machines - identical code
will fail at different points on two different machines.
3.	The error is not related to contributed packages, because our code
doesn't use any. The code *does*, however, use repeated calls to
optim() and nlm(), and passes several arguments through a sequence of
functions using "...".
4.	Occasionally, we get an error relating to subset replacement
instead of "Error:bad value". For example:

x <- rnorm(10)
x[1] <- 3

Error in x[1] <- 3 : could not find function "[<-"

5.	The error behaviour changes as a result of minor modifications to
print() statements in the code e.g. by inserting a line that prints
the value of a well-defined variable.

I suspect this really *is* a bug (particularly since it only started
happening with version 2.5.0), but I figure it would be worth giving
people a chance to tell me I'm an idiot before reporting it as such.
In case anybody would like to try and see the error for themselves, I
have uploaded some files to
http://www.homepages.ucl.ac.uk/~ucakarc/Rtest/. The file ErrorDemo.r
is the main script - see the file header there for more details.
Files momfit.r and elmstats.dat are also required for this example to
work. I'm sorry that it isn't a very simple example, but I haven't
seen a simple illustration of the problem (and I couldn't find any
examples in the list archives either).