I have to say that I'm bemused by the amount of effort that people will go to just to avoid coming up with hostnames for their machines. (This is different from the lazy people, who just don't bother creating PTR records at all.)

By now I am used to the people who carefully give their IP addresses a PTR record of the IP address itself:

2.0.0.127.in-addr.arpa. PTR 127.0.0.2.

(Naturally these show up as invalid reverse DNS, since there is no corresponding A record.)

I see enough of these that my tool for doing reverse DNS lookups prints them specially. I'm never sure if these are deliberate or just the result of a well intentioned person being told that all of their IP addresses should have PTR records, whether or not they have hostnames.

Recently, though, someone did this one better: they gave their IP addresses PTRs to themselves in the .in-addr.arpa zone and then gave them corresponding A records; the result was entirely valid hostnames like 35.95.2.81.in-addr.arpa. The sheer blank Zen-like perfection of this perfectly valid yet completely useless non-answer still stuns me, especially since they had to go to extra work to add the A records. (But at least they have valid reverse DNS.)

(Also, I would be remiss if I didn't give an honorable mention to as15444.net, a domain that is named after their ASN.)