Carl Malamud, a well respected technologist, author, and public domain advocate posted on Twitter that Bing's spider "got a timeout" by not playing nice with the IRS database. It seems that BingBot was a bit too aggressive on their site and they decided to block Bing for some time.
Here is the tweet:
Bing got a timeout on the IRS database today. Bing was a bad robot. Bing is sitting in a corner, thinking about what it means to play nice.— Carl Malamud (@carlmalamud) April 25, 2014
This caught the attention of Google's Matt Cutts and it drove Carl to add more details:
@mattcutts my server isn't huge, fine with well-behaved bots, they got all piggish. (we ban your ftp robot, but for different reasons.)— Carl Malamud (@carlmalamud) April 25, 2014
It isn't clear exactly which site Carl decided to block. He referenced the IRS database. Bug clearly he is calling BingBot a not well-behaved bot, at least not on April 25th.
Forum discussion at Twitter.