I did something similar to this once, forgot to bypass the H2O softener prior to brewing an American Brown ale. I didn't realize it until the end of the brewing session. My efficiency was much lower than expected which puzzled me at the time. Went ahead and pitched the yeast, fermented and kegged. While the beer was obviously different than prior batches of the same recipe, thinner and interestingly enough not as roasty as expected, it was still drinkable and did not have any undesireable attributes. One would assume that for an ABA extremely soft water would be a bad thing. Go figure.