Even in the self-test suite, the bug appears, run:

python /usr/lib/python2.4/curses/textpad.py

then type so that the line wraps, eg 123456789123456789 to fill two 
lines, and then ctrl-g to return -- the result has had the end 
characters of each line removed, so the return value is 
12345678\n12345678\n

As a quick hack on my local install I changed line 56 from:

last = min(self.maxx, last+1)

to

last = min(self.maxx, last)+1

and it seems to work, but I have no idea if this is the right way to 
fix it (ie, this might have side effects, or this off-by-one might 
happen in several places and need a global fix, not just one local one)

Also present in 2.5

This doesn't happen to me with 2.6 compiled from HEAD, or with 2.5.2
shipped with Ubuntu Hardy.

In both cases the output is:

Contents of text box: '123456789\n123456789\n'

This bug was fixed in 2.5 and 2.6, in rev. 60118 and 60119.  Thanks for
your bug report!