כי אתה סלחן לישראל ומחלן לשבטי ישראל (יום כיפור)
Most of the time HaShem is referred to with the adjectives סולח ומוחל. Here the words have an addition of a closing Nun.
The Gemara (Bava Metziah 33a) says on the verse (Shmos 23, 5) that when one sees the donkey of your friend burdened (רובץ) under its load, then one should surely assist, that the verse using the word רובץ and not the word רבצן to indicate when one is obligated. Rashi explains that a רבצן implies that it is regularly burdened, and then one would be exempt, since it is like a normal burden to that donkey, and not an unusual, heavy burden needing outside assistance.
Thus, the closing Nun teaches two concepts. One, it strengthens and reinforces the adjective. Second, it indicates a consistency. Thus, we allude to HaShem that he is accustomed to forgive with strength the many sins of the Jews. Also, HaShem has an attribute of constantly forgiving.
