Error: Missing \endcsname inserted

Apparently, this is a quite unspecific TeX error. Sometimes the proposed solution is really hardcore, but still, it seems like the general idea is that this error is a sign that somewhere, you misplaced some programming punctuation, like, as shown in my first link, an extra backslash.

I’ll only cover here the case which I just encountered: the LaTeX template of CAp 2012 (“Conférence Francophone sur l’Apprentissage Automatique”), a French conference on Machine Learning, was broken (even the provided PDF was…). With the provided template, I’d get about 30 errors, the first one being one of those “Missing \endcsname inserted”. That first error occurred at the first citation insertion, \citep{Charlet:JAC94}, which referred to the following BibTeX entry:
@InProceedings{Charlet:JAC94,
author = "Jean Charlet and Bruno Bachimont and Jacques Bouaud
and Pierre Zweigenbaum" ,
title = "Ontologie et r\'eutilisabilit\'e : exp\'erience et
discussion",
pages = {C1--C14},
booktitle = {Actes des JAC'94},
year = 1994
}
It got me skeptical for a (long) while. Then I though I’d just apply some basic programming principle: don’t f*** with variable names. The name of that bibliographic entry just looked crappy: a column in a variable name, what the heck?

… and removing the columns in all bibliographic entries fixed all of it, indeed. Like this:
\citep{CharletJAC94}
and:
@InProceedings{CharletJAC94,
author = "Jean Charlet and Bruno Bachimont and Jacques Bouaud
and Pierre Zweigenbaum" ,
title = "Ontologie et r\'eutilisabilit\'e : exp\'erience et
discussion",
pages = {C1--C14},
booktitle = {Actes des JAC'94},
year = 1994
}