an unexpected result in LaTeX

[Kevin Cahill]

The code 

\begin{equation}
\boldsymbol [ \c_b, \c_c \boldsymbol ]^\dag
={} [ \c_c^\dag, \c_b^\dag ]
= \left\{
\begin{array}{ll}
- \boldsymbol [ \c_b, \c_c \boldsymbol ] & 
\mbox{ for } b, c = 1, 2, 3 \\
\boldsymbol [ \c_b, \c_c \boldsymbol ] & 
\mbox{ if } b \mbox{ or } c = 0 . 
\end{array} 
\right .
\end{equation} 

works, but the nearly identical code

\begin{equation}
\boldsymbol [ \c_b, \c_c \boldsymbol ]^\dag
={} [ \c_c^\dag, \c_b^\dag ]
= \left\{
\begin{array}{ll}
- \boldsymbol [ \c_b, \c_c \boldsymbol ] & 
\mbox{ for } b, c = 1, 2, 3 \\
[ \c_b, \c_c ] & 
\mbox{ if } b \mbox{ or } c = 0 . 
\end{array} 
\right .
\end{equation} 

does not;
the error message is

418:  Illegal unit of measure (pt inserted).
<to be read again>
                           _
l. 418 [ \c_b, \c_c ]
                             &
?
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