I just found out that I cannot (with MkIV on Windows XP, ConTeXt  ver.
2011.02.09 16:42) compile the following

\starttext
This formula produces an error:
\startformula
[r_{\rm test}]=a
\stopformula
\stoptext
\starttext
However, this works:
\startformula
a=[r_{\rm test}]
\stopformula

What's going on here? Using \mathrm instead of \rm in the subscript is
compiling without complaint, but produces the wrong output.

Julian