http://www.purl.org/tcl/home/man/tcl8.4/TclCmd/tclvars.htm 

[[Someone want to comment on where this global variable
gets set and to what good uses one can put it to?]]

[RS]: Contains the error message and traceback of the last error. Silly example:
 % proc foo {} {bar x}
 % proc bar {input} {grill$input}
 % foo
 invalid command name "grillx"
 % set errorInfo
 invalid command name "grillx"
    while executing
 "_unknown grillx"
    ("eval" body line 1)
    invoked from within
 "eval _unknown $args"
    (procedure "unknown" line 17)
    invoked from within
 "grill$input"
    (procedure "bar" line 1)
    invoked from within
 "bar x"
    (procedure "foo" line 1)
    invoked from within
 "foo"
This tells us that [unknown] raised the error when trying to resolve ''grillx'', called in ''bar'', called in ''foo''. (It does not tell us that the real bug was a missing space between ''grill'' and ''$input''.. that's up to the coder to find out. But the traceback may help to narrow in on the bug, at least).

See also [errorCode].
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[Tcl syntax help]