For some reason couple of the thick vertical lines are missing from the first row (still there is the thin line):
\documentclass{article}
\usepackage[utf8]{inputenc}
\usepackage{multirow}
\usepackage{bigstrut}
\usepackage{adjustbox}
\usepackage{makecell}
\usepackage{amssymb}
\usepackage{array}
\newcolumntype{?}{!{\vrule width 2pt}}
\newcolumntype{M}[1]{>{\raggedleft\arraybackslash}m{#1}}
\begin{document}
\renewcommand{\arraystretch}{0.66}
\begin{table}[htbp]
\centering
\caption{Review of methods and ...}
\begin{adjustbox}{center}
\setlength\tabcolsep{2pt}
\begin{tabular}{|c?c|c?c|c|c|c|c?c|c|c|}
\hline
& \multicolumn{2}{c|}{\linespread{1}\selectfont{}\centering Bugs' info} & \multicolumn{5}{c|}{Expertise info} & \multicolumn{3}{c|}{Based on} \\
\hline
Method & \parbox{1.7cm}{\linespread{1}\selectfont{}\centering Title + description} & \parbox{0.8cm}{\linespread{1}\selectfont{}\centering Meta} & \parbox{1.9cm}{\linespread{0}\selectfont{}\centering Bug fixing; title / description} & \parbox{1.6cm}{\linespread{1}\selectfont{}\centering Being a committer} & \parbox{1.1cm}{\linespread{1}\selectfont{}\centering Tossing history} & \parbox{0.8cm}{\linespread{1}\selectfont{}\centering Meta} & \parbox{1.3cm}{\linespread{1}\selectfont{}\centering Changed code} & \parbox{1.6cm}{\linespread{1}\selectfont{}\centering Final committer} & \parbox{1.3cm}{\linespread{1}\selectfont{}\centering Closer / resolver} & \parbox{1.3cm}{\linespread{1}\selectfont{}\centering Drafted assignee} \\
\hline
AA & \checkmark & & \checkmark & & & & & \checkmark & \checkmark & \\\hline
BB & \checkmark & & \checkmark & & & & & \checkmark & \checkmark & \\\hline
CC & \checkmark & \checkmark & \checkmark & & \checkmark & \checkmark & & \checkmark & \checkmark & \\\hline
\end{tabular}%
\end{adjustbox}
\label{tab:previousMethods}%
\end{table}%
\end{document}
?
spec by a|
by using\multicolumn{2}{c|}
I assume you intended\multicolumn{2}{c?}
– David Carlisle May 23 '17 at 22:24\linespread
and\centering
do nothing in ac
cell as that is horizontal mode like\mbox
– David Carlisle May 23 '17 at 22:25