Last Friday, the US bombed a hospital run by Médecins Sans Frontières (MSF)  in Kunduz, destroying it in a nearly hour-long raid with many of its patients and doctors still inside.
On Saturday, the official statement was that this had been a mistake, a mistargeting in the fog of war.
On Sunday, MSF pointed out that not only did they routinely give the GPS coordinates of the hospital to US forces (most recently five days before the bombing), but they had been on the phone with military officials during the strike
trying to get them to stop, with the US nonetheless continuing to bomb for another half hour, and that on top of this the other buildings in the compound were undamaged even as just the hospital was destroyed.
On Monday, the US rolled back its story, claiming that the strike had been deliberate, but had been necessary because the Taliban was using the hospital as a base and had been firing out of it, pinning down US and Afghan forces and requiring close air support.
On Tuesday, as film reviewed by the AP demonstrated that this was not, in fact, the case, and that nobody had been firing anything from the hospital, the US rolled back its story yet again, and is now claiming that while there were a lot of civilian casualties in the hospital, the attack was nonetheless justified because there were a lot of senior Taliban members in
the hospital who were also killed.
As MSF's General Director, Christopher Stokes, said on Sunday, these latest official US statements "[amount] to admission of a war crime;" at this point, the statements amount to "yes, we blew up a hospital with people in it, and yes, we did that on purpose, but we were justified in doing so."
[Edited to add: And on Wednesday, the administration admitted fault and promised a "transparent, thorough, and objective accounting" for what happened. h/t +Cory Lui
What seems to be the case, and what most sources have started to concede, is that the Afghan government has long hated the existence of this particular hospital, as (like all MSF hospitals) it has been known to treat whoever was sick or injured and came through the door, no matter which side they were on.
My best estimate of the underlying reasoning, at this point, has to do with the Obama administration's ongoing negotiations with the Afghan government about the pullout of remaining US forces, and their urge to make extra-nice with the Afghans -- say, by blowing up some hospitals for them -- to try to keep them on our side for a bit longer after we leave.
The article linked provides an excellent timeline, as well as quotes from all sides as the story unrolled, so that you can see how the official statements have shifted as each line became untenable in turn.
 Disclosure: You'll find me on their major donors list for the past decade.
[This post has been edited with a substantial rewrite for clarity, since I wrote the first draft half-asleep.]