A Microsoft spokesperson tells me that "accidental human error" is to blame for missing images of "tank man" on its Bing search results.
"We are actively working to resolve this."
The incident comes on the 32nd anniversary of the Tiananmen Square Massacre.
It seems unlikely that you could accidently censor something like this globally without trying to do it for at least one specific target demographic.
It's also plausible that the fault was brought in deliberately by a rogue engineer to raise the subject globally.