Can't set debug-on-error to nil

[Yevgeniy Makarov <emakarov@gmail.com>]

Hello,

In my Emacs sessions, the variable debug-on-error is always t, so
every error pops a buffer with "Debugger entered--Lisp error", etc. I
found the following:

* M-x describe-variable debug-on-error says that its value is nil, but
M-: debug-on-error prints t

* This happens even if I start Emacs with -Q option

* I tried putting (setq debug-on-error nil) in the beginning and in
the end of .emacs, as well as customizing the variable to nil (there
is an entry '(debug-on-error nil) in custom-set-variables), but it did
not help.

* There is nothing about debug in my .Xdefaults

So where is it set to t, and how can I set it to nil?

I have GNU Emacs 23.0.60.2 of 2008-01-25 on Linux.

Thank you,
Evgeny