David Carlisle explained why \hspace{0pt}
makes such a difference. I'd like to explain how one could change the behaviour of accents over \mathcal{C}
so that they're not that much off to the right. In fact, I'd go as far as to say that there's an error in the font metrics of the \mathcal
font cmsy10
. My solution works on UNIX type systems; here's a comparison of the unpatched versus patched font metrics.

I think that the patched version in the right looks much better. To produce the patched font metrics, run the following shell script in your current TeX directory:
for i in 5 6 7 8 9 10
do tftopl $(kpsewhich cmsy$i.tfm) |
sed '
/(LABEL O 103)/d
/(LABEL O 104)/a\ \ \ (LABEL O 103)' > modcmsy$i.pl
pltotf modcmsy$i.pl
rm modcmsy$i.pl
done
This will produce 6 files modcmsy5.tfm
to modcmsy10.tfm
. (If you think that the accents are still too far to the right, replace 104
with 113
in the shell script¹.) Now the LaTeX file
\documentclass[12pt]{article}
\DeclareFontShape{OMS}{cmsy}{m}{n}{%
<5><6><7><8><9><10>gen*modcmsy%
<10.95><12><14.4><17.28><20.74><24.88>modcmsy10%
}{}
\pdfmapline{+modcmsy5 CMSY5 <cmsy5.pfb}
\pdfmapline{+modcmsy6 CMSY6 <cmsy6.pfb}
\pdfmapline{+modcmsy7 CMSY7 <cmsy7.pfb}
\pdfmapline{+modcmsy8 CMSY8 <cmsy8.pfb}
\pdfmapline{+modcmsy9 CMSY9 <cmsy9.pfb}
\pdfmapline{+modcmsy10 CMSY10 <cmsy10.pfb}
\usepackage{amsfonts}
\begin{document}
$\dot{\mathcal C}$ $\tilde{\mathcal C}$ $\widehat{\mathcal C}$
\end{document}
will produce as output the right part in the above image, and the accent correction will work for all font sizes. (Thanks, egreg, for telling me about \pdfmapline
!) Personally, I don't like the \widehat
from amsfonts
so much, so I'd just omit \usepackage{amsfonts}
if possible:

¹Just a short explanation: 103
(octal number!) is the position of \mathcal{C}
in the font. In the original font metrics, accents over \mathcal{C}
are shifted to the right by the same amount as over \mathcal{B}
. In the patched version it's the same as over \mathcal{D}
(104
in base-8); the alternative 113
corresponds to \mathcal{K}
.