Aligner can help find cases where alignment has slipped (one sentence having been translated as two or three). One method is to use the menu item Match by Capitals. This searches for matching proper nouns in the two versions: if say Paris is mentioned in sentences 25 of the source text and not in sentence 25 of the translation but in sentence 27, it is very likely that some slippage has occurred.
Viewer & Aligner will search forwards from the current text sentence on, and will tell you where there's a mis-match. You should then search back from that point to find where the sentences start to diverge. It may be useful to sample every 10 or every 20 to speed up the search for slippage.