I just discovered that mtr has some problems with IPv6 enabled hosts when using the GUI. Try the following:
1) enter "mtr ipv6-host" in shell -> works
2) enter then an IPv4 only host in Hostname input line -> doesn't work
3) quit mtr
4) enter "mtr ipv4-host" in shell -> works
5) enter then an IPv6 enabled host in Hostname input line -> tracing IPv4 part of IPv6 enabled host
So, something is broken, at least for 2). One could argue if 5) might be OK or the expected behaviour, but I believe that mtr should query the AAAA records and trace for the IPv6 hostname, regardless of whether you started with an IPv4 only host first.
Is there already someone tracking IPv6 related bugs in Debian? A tag in BTS maybe? Anyway, the above is reported as #521415.
UPDATE: added blog title ;)