http://purl.org/tcl/home/man/tcl8.4/TclCmd/bgerror.htm
is the official man page.

[[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.

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[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 
 }


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[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

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