Doubts about EvalPlanQual

Hi,

When I read function "EvalPlanQual", I found the following code:

if (heap_fetch(relation, &SnapshotDirty, &tuple, &buffer, true, NULL))
{
/*
* If xmin isn't what we're expecting, the slot must have been
* recycled and reused for an unrelated tuple. This implies that
* the latest version of the row was deleted, so we need do
* nothing. (Should be safe to examine xmin without getting
* buffer's content lock, since xmin never changes in an existing
* tuple.)
*/
if (!TransactionIdEquals(HeapTupleHeaderGetXmin(tuple.t_data),
priorXmax))
{
ReleaseBuffer(buffer);
return NULL;
}

AFAICS, when Vacuum decides to reclaim any version V of a tuple T, there
must be none concurrent transactions that are accessing or will access any
versions before V, because HeapTupleSatisfiesVacuum ensures this.

If I'm right, then my doubt is: how can the branch "if
(!TransactionIdEquals(HeapTupleHeaderGetXmin(tuple.t_data), priorXmax))"
happen? Is this a dead branch?

If not, can anyone give an example to explain how does this happen?

Thanks a lot.