I'm working on the data dump of stackoverflow and I might have found a bug in it.
Id post1 = 1406604 with AcceptedAnswersId = 1406611
Id post2 = 784929 with AcceptedAnswersId = 784946
And Id answer = 1406611 have ParentId = 784929
The problem is that the AcceptedAnswerId of the post1 is still here.
In an other post I remark that the post for which its AcceptedAnswer has been removed keep a value for its AcceptedAnswerId. link3
Maybe a modification can be made.