documentclass{article}
usepackage{tabularx}
begin{document}
  noindent
  begin{tabularx}{linewidth}{ X | X }
    centering A & B \ hline
    1.0 & 2.0 \
  end{tabularx}
end{document}

documentclass{article}
usepackage{tabularx}
begin{document}
  noindent
  begin{tabularx}{linewidth}{ X | X }
    centering A & centering B \ hline
    1.0 & 2.0 \
  end{tabularx}
end{document}

! Misplaced noalign.
hline ->noalign
                  {ifnum 0=`}fi hrule @height arrayrulewidth futurelet...
l.8   end{tabularx}

! Extra alignment tab has been changed to cr.
<recently read> endtemplate

l.8   end{tabularx}

! Misplaced noalign.
hline ->noalign
                  {ifnum 0=`}fi hrule @height arrayrulewidth futurelet...
l.8   end{tabularx}

! Extra alignment tab has been changed to cr.
<recently read> endtemplate

l.8   end{tabularx}

! Misplaced noalign.
hline ->noalign
                  {ifnum 0=`}fi hrule @height arrayrulewidth futurelet...
l.8   end{tabularx}

! Extra alignment tab has been changed to cr.
<recently read> endtemplate

l.8   end{tabularx}