\documentclass{article}

\usepackage{mytest}
\usepackage{mdframed}
\usepackage{xcolor}
\begin{document}

C-c C-m test-one RET works.

C-c C-m \test-two RET throws:
Debugger entered--Lisp error: (wrong-type-argument sequencep mytest-left)
  append(mytest-left mytest-right)

C-c C-m test-three RET works.

C-c C-m test-four RET works.

C-c C-m test-five RET works.

C-c C-m test-six RET works.

\end{document}

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