an unexpected result in LaTeX

[barbara beeton]

One of the features of the line-breaking code \\ is the ability to
add an option -- enclosed in [brackets] as the recognized syntax --
following the \\.  The required content is a dimension, such as [2pt].

That is what you have inadvertently stumbled upon.

To protect against this misunderstanding, insert an empty group {}
or \relax after \\ if the next thing is to be an "ordinary" bracketed
expression.

This can also happen in some situations in the first line (such as
in a table), so you probably want to become familiar with this
concept and the proper method of repair.
 						-- bb

On Wed, 7 Jul 2021, Kevin Cahill wrote:

> 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 ]
>                              &
> ?
> 
>