http://purl.org/tcl/home/man/tcl8.4/TclCmd/bgerror.htm
is the official man page. As of Tcl 8.5, [interp bgerror] provides 
a more flexible handling of these things.

    :   '''bgerror''' ''message''

Note that it is ''not'' expected that user code will call this directly; instead, it is there to be redefined so that user code can monitor and respond to errors in event handlers.

    :   ''Note that in very old code you may see this command written as `tkerror`; that name is obsolete nowadays. Use '''bgerror''' instead as it has the same semantics but without the implicit coupling to Tk.''

<<discussion>>

[[fill in useful tricks and examples for using bgerror]]

Does the documentation describe the calling convention?  I didn't notice
it.  My experience is that the error-handler invokes it as something like
      bgerror [firstLineOf $::errorInfo]

During development, one might rush a hack like
======
      proc bgerror message {
          puts stderr "An event-based script faulted with '$message'."
      }
======
into use.

[DKF]: I find that it's better to print `[errorInfo]` in such cases.
----

[Ken]: I want to enquire what is the best method if i got a proc that runs every 20s like the follow code but i want to catch its errors if any of the proc that run afterwards run into errors? I heard using bgerror is not recommended, so what is the best method?
======
 proc a { } {

  [do something]

  after 1000 a 
 }
======

----
[KPV] 2008/06/13 : NB. in 8.4 the default bgerror code is not [tile] 
aware. Thus, the following idiomatic (but incorrect) code will fail
(more precisely, the bgerror code configures a scrollbar's -relief 
which tile doesn't allow):
======
  package require -exact Tk 8.4
  package require tile
  namespace import -force ::ttk::*
  after 10 asdf
======
<<discussion>>
----
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