The limits of the use of accents package

Question

We suppose to have this minimal compilable code:

\documentclass[a4paper,12pt]{article}
\usepackage{mathtools,amssymb}
\usepackage{accents}
\usepackage{bm}

\begin{document}
$\accentset{\overset{\overset{\sim}{\square}}{c}}{\bm{\mathrm{x}}}(t)$
\end{document}

Inside the first curly bracket \overset{\overset{\sim}{\square}}{c} of the command \accentset I can put a castle of symbols.

Why this package give me several errors when I use \tilde or \dot?

What characters should be deleted?

MWE that it does not compile:

\documentclass[a4paper,12pt]{article}
\usepackage{mathtools,amssymb}
\usepackage{accents}
\usepackage{bm}

\begin{document}
$\accentset{\overset{\overset{\tilde}{\dot}}{\sim}}{\bm{\mathrm{x}}}(t)$
\end{document}

The file .log it is very long.

Answer 1

It isn't clear what the intention of \overset is here. There are two accent mechanisms involved, commands that are defined as accents, that take an argument, like \tilde. Then commands constructed with \accentset that can be used with a symbol to fake an accent taking the symbol and placing it over the base. So to stack \tilde, dot and \sim you just need

\documentclass[a4paper,12pt]{article}
\usepackage{mathtools,amssymb}
\usepackage{accents}
\usepackage{bm}

\begin{document}
$\tilde{\dot{\accentset{\sim}{\bm{\mathrm{x}}}}}(t)$
\end{document}

Although \bm{\mathrm{x}} could more easily (and efficiently) be written as \mathbf{x}

Answer 2

You can overcome the compilation errors by using the \mathchars associated with \tilde and \dot.

\documentclass[a4paper,12pt]{article}
\usepackage{mathtools,amssymb}
\usepackage{accents}
\usepackage{bm}
\begin{document}
$\accentset{\overset{\overset{\mathchar"307E}{\mathchar"05F}}{\sim}}{\bm{\mathrm{x}}}(t)$
\end{document}

However, given the poor spacing and sizing, one might consider a stack alternative:

\documentclass[a4paper,12pt]{article}
\usepackage{mathtools,amssymb}
\usepackage[usestackEOL]{stackengine}
\stackMath
\usepackage{bm}
\begin{document}
$\setstackgap{S}{-5pt}
\stackon[-1pt]{\bm{\mathrm{x}}}
  {\Shortstack{\mathchar"307E\\\mathchar"05F\\\scriptscriptstyle\sim}}(t)$
\end{document}