"vim" has quit unexpectedly

[Marc Weber]

How would you try to track this error down?
Causing vimscript code:

        echo "tag_text is"
        echo tag_text
        echo "if"
        echo len(tag_text)
                if len(tag_text)>0
                echo "in if"

output:

        tag_text is
        []
        if
        0

Then vim either hangs, crashes and occasionally shows some internal error..
the shell shows this ugly string:
*** glibc detected *** free(): invalid pointer: 0x084b1f0f ***

This might be caused anywhere, right?
Would a calling stack trace or output of gvim -Vfile help?

Marc