The following example fails to compile, complaining "Missing { inserted."

It compiles cleanly  if one or more of the three marked lines are removed.

This is the case with TL2013 and with recent standalone beta versions. I 
do not know if it ever worked.

    \setuplayout [grid=yes]%                            line one
    \definetextbackground[TB][
                      topoffset=.25em,bottomoffset=.25em,% line two
                      location=paragraph,%                 line three
                     ]
    \starttext
    \startTB
       prisca iuvent alios ego me nunc denique natum gratulor: haec
       aetas moribus apta meis.
    \stopTB
    \stoptext

-- 
Rik