I was asked today if one could make LaTeX skip all occurances of the value <b>13</b>, in all counters, as <a href="http://en.wikipedia.org/wiki/Thirteenth_floor">some buildings do</a>. Well, of course you can:
```latex
\let\stepc\stepcounter\renewcommand{\stepcounter}[1]{%
\ifthenelse{\equal{\value{#1}}{12}}{\stepc{#1}}{}\stepc{#1}}
```
Here, we first save the `\stepcounter` macro in `\stepc`, then we renew its definition by stepping the counter twice if its current value equals 12, therefore effectively omitting the 13th occurance. Also, you should watch <a href="http://www.imdb.com/title/tt0139809/?ref_=fn_al_tt_1">the thirteenth floor</a>, it's a nice movie if you're into scifi.