I think this is a bug. If it's in Tex itself
Donald K. owes someone a dollar. :-)
-burt
=====================================
\documentclass[11pt]{article}
\begin{document}
\[
f(x) = \left \{
\begin{array}{lll}
[4,5] & x\in [0,2) & \\
% BEGIN SECTION OF INTEREST
% bug
%
[0,1] \cup [4,5] & x\in[2,3] & \\
[0,1] & x \in (3,5] & \\
%
% error reads:
%! Illegal unit of measure (pt inserted).
%<to be read again>
% >
%l.14 [0,1]
% \cup [4,5] & x\in[2,3] & \\
%
% bug workaround
% {} [0,1] \cup [4,5] & x\in[2,3] & \\
% {} [0,1] & x \in (3,5] & \\
%
% END SECTION OF INTEREST
\end{array}
\right.
\]
\end{document}
==============================================
