LaTeX: optional arguments with square brackets

Note: Without a MWE I will remove \end{tabular} from the definition of your macro and ignore the fact that you declare two argments (one optional, one mandatory) but use only #1.

An optional argument behaves quite different from normal arguments and groups as it is catched by TeX with the help of the [/] delimiters.

With a macro is defined with

\newcommand{\monkeyBeans}[2][]{\parbox{4in}{#1}}

and called with

\monkeyBeans[$[a, b]$]{yo monkey}

#1 will be $[a, b. #2 will be $ and in this case discarded as #2 is not used in your macro definition. Therefore, \monkeyBeans[$[a, b]$]{yo monkey} is expanded to

\parbox{4in}{$[a, b}]{yo monkey}

No wonder there's a $ missing! I'd miss it, too.

With a macro \monkeyBean defined as

\newcommand{\monkeyBean}[1][]{\parbox{4in}{#1}}

and used with

\monkeyBean[$[a, b]$]yo monkey

It's even clearer what happens, as TeX complies twice about an missing $ because that line expands to

\parbox{4in}{$[a, b}$]yo monkey

and “]yo monkey” is typeset in math mode, too.

---

Solution: Enclose optional arguments with a ] in braces, so that they get grouped:

\monkeyBean[{$[a, b]$}]
\monkeyBeans[{$[a, b]$}]{yo monkey}

Qrrbrbirlbel's solution is the standard LaTeX2e approach, and is certainly what you have to do for an arbitrary command. The LaTeX3 package xparse deals with the nesting in a cleaner way:

\documentclass[letter,12pt]{article}
\usepackage{xparse}

\NewDocumentCommand{\monkeyBeans}{O{}m}{%
  \parbox{4in}{#1}%
}

\begin{document}

\parbox{4in}{$[a, b]$} 

\monkeyBeans[$[a, b]$]{yo monkey}

\end{document}

which works of course only if you are defining the command where [] nesting is required.