[Tcl8.5a2/Cygwin] What is "BAD ASSUMPTION"?

Very strange thing.

Version: tclsh8.5a2
OS: Cygwin (homecooked with manually corrected problem with dlls - maybe you
remember)

The scripts works correctly, behaves as I expected, produces expected results.
With only one "small" disturbance - this text is printed on stderr:

BAD ASSUMPTION

I tried to select the fragment of code, in which it happens, believing that
this text is not queued anywhere, but printed directly when something wrong
happens.

This is a small fragment, what is in this place:

proc processlines text {

        set lines [split $text \n]
        ...
}

foreach par $parlist {
        set par [do some replacements with $par]

        set linesout [processlines $par]
...
}

I placed marks in two places. First is just before the instruction, where
[processlines] is called. Second mark was the first command in [processlines].
The "BAD ASSUMPTION" text is printed between them.

First I thought that there is something wrong with the length of text or its
contents, but no - this just happens first time the [processlines] is called.

The script is a bit long, so I wouldn't like to send it on the group. The
script itself is also not enough; you would require also the input text it
converts. Please let me know if this is a known problem (resolved/to be
resolved) or this is something unknown; I'll send both script and the input
text to the interrested.

Regards,

-- 
//  _    ___         Michal "Sektor" Malecki <sektor(whirl)kis.p.lodz.pl>
\\ L_ |/ `|  /^\ ,()                         <ethourhs(O)wp.pl>
// \_ |\  \/ \_/ /\ C++ bez cholesterolu: http://www.intercon.pl/~sektor/cbx
"Java does not have pointers!"