One way to verify sites when using Googles webmaster tools is to add a TXT entry to the nameserver, using the hostname and adding the provided text.
Accidentally (because I simply didn't know) I added two TXT entries, one with 'www' and one without, just to make sure Google would accept the code (because in the webmaster tools the site was entered with 'www').
What happened was: after the DNS entries had spread, the sites were not available anymore when using the hostname with 'www'. EDIT: No particular error message, just "Server not found".
Why? It was probably naive to think that the TXT entry format is somewhat arbitrary, but can someone explain to a shocked developer (=non-administrator) why a TXT record can influence something that should be handled by A records so destructively?
EDIT: Here is the example (but of course it is no longer online). The site is hosted at domainfactory, a shared hoster where one can edit the DNS settings (or have domainfactory manage them - in that case, the 'Ziel' column shows their name; usually it is no problem to mix entries):
It is exactly the last entry that made the site unavailable.
However, telling me that this should not happen is also a good answer - I can ask the hosting provider then.
Forgive me if I'm not familiar with nslookup, but I did one lookup with and one without www on one of the domains that still don't work, and this is the result:
C:\>nslookup www.foo.de Server: dns2.colt1.inetserver.de Address: 18.104.22.168 Name: www.foo.de
And the second one:
C:>nslookup foo.de Server: dns2.colt1.inetserver.de Address: 22.214.171.124 Nicht autorisierende Antwort: Name: foo.de Address: 126.96.36.199
The difference is that the request without 'www' showed a 'Nicht autorisierende Antwort:' (probably 'non-authorizing answer') but the correct IP.