no blocks must be born in a txg after a snapshot is created
No new blocks must be born in a dataset in the same TXG after a snapshot of the dataset is taken.
Those blocks would have the same
blk_birth as the dataset's
ds_prev_snap_txg and as such they would be presumed to belong o the snapshot while in fact they do not.
All the datasets must be clean before sync tasks are run, so the described scenario may happen only if one of the sync tasks dirties the dataset and another sync task takes its snapshot.
Then, there will be another sync pass because of the dirty data and the new blocks will be born in the same TXG when the data is written out.
It seems that almost all of the existing sync tasks modify only MOS and do not dirty any objsets.
The only exception that I've been able to identify so far is the rollback which can modify an objset when it zeroes out the objset's ZIL.