documentclass[border=10pt]{standalone}
usepackage{tikz}
begin{document}
begin{tikzpicture}
    path (0,0) node[draw]{boldmath (+)} (1,1);
end{tikzpicture}
end{document}

! Package tikz Error: Giving up on this path. Did you forget a semicolon?.

See the tikz package documentation for explanation.
Type  H <return>  for immediate help.
 ...

l.6     path (0,0) node[draw]{boldmath (+)}
                                                (1,1);
?

path (0,0) node[draw]{{boldmath (+)}} (1,1);