There seems something very much amiss with \halign in later ConTeXt versions.
This typesets fine in PlainTeX and is an example taken from a textbook.
\tabskip=1em\halign{% \hfil\it#\hfil&\hfil#\hfil&#&\hfil#\crA&B&C&D\cr}
Also in contextversion 2012.05.30 (from a TeXlive distribution).
But it fails at least in ConTeXt ver: 2013.03.20 10:34 MKIV and in ConTeXt ver: 2013.04.16 12:08 MKIV beta with the following error
! Only one # is allowed per tab. system > tex > error on line 5 in file fixedwidth.tex: Only one # is allowed per tab ...
1 \starttext 2 contextversion=\contextversion\par 3 \tabskip=1em 4 \halign{% 5 >> \hfil\it#\hfil&\hfil#\hfil&#&\hfil#\cr 6 A&B&C&D\cr} 7 \stoptext l.5 \hfil\it#\hfil&\hfil# \hfil&#&\hfil#\cr
Why? How primitive it may be, I would like to use \halign now and then.
Hans van der Meer