However, in a 2 s , w r became erroneously aligned, because the alignment distribution 3 of w r was only learned from a 1 s , thus consisted of non-zero values only for generating the target-side words in s. Therefore, the alignment probabilities from the rare word w r to the unaligned words in s were extraordinarily high, since almost all of the probability mass was distributed among them.
the story behind these garbage collector effects is that erroneous alignments are able to provide support for themselves; the probability distribution learned only from s is re-applied to s. In this way, these "garbage collector effects" are a form of over-fitting.