documentclass{beamer}
begin{document}
begin{frame}
frametitle{What's going on here?}
xdefmytext{underline{duck}}
underline{underline is not the problem}
end{frame}
end{document}

! Use of beamer@@frametitle doesn't match its definition.
new@ifnextchar ...served@d = #1def reserved@a {
                                                  #2}def reserved@b {#3}f...
l.7 end{frame}

documentclass{beamer}
begin{document}
begin{frame}
frametitle{What's going on here?}
xdefmytext{noexpandunderline{duck}}
underline{underline is not the problem}
end{frame}
end{document}