No DNS poisoning at Google (in case of trouble, blame the DNS)

On Wed, Jun 27, 2012 at 03:53:17AM +0000, Matthew Black <Matthew.Black@csulb.edu> wrote a message of 18 lines which said:

We believe the DNS servers used by Google's crawler have been poisoned.

[After reading the whole thread and discovering that Google was indeed right.] What made you think it can be a DNS cache poisoning (a very rare event, despite what the media say) when there are many much more realistic possibilities (<troll>specially for a Web site written in PHP</troll>)? What was the evidence pointing to a DNS problem?