The bar construction is a functor $A\mapsto Bar(A)$ from the category of augmented differential graded algebras over a commutative ring $R$ to the category of chain complexes of $R$-modules. It sends an algebra $A$ to the derived tensor product $R\otimes^{L}_AR$.
Assume that I have a map of augmented algebras $A\to B$ which induces a quasi-isomorphism $Bar(A)\to Bar(B)$. This does not necessarily implies that the original map was a quasi-isomorphism but I believe it does if some connectivity or co-connectivity assumptions are made on $A$ and $B$. Philosophically speaking, the bar construction is the algebraic analogue of the loop space construction for based spaces and this construction is conservative if one restricts to connected based spaces.
I am using the homological grading convention. My guess is that the Bar construction is conservative when restricted to simply coconnected algebras (i.e. whose homology is concentrated in degrees $\leq -2$ apart from the unit in degree zero). The analogy with spaces suggests that this might actually also be true for coconnected algebras but I'm not so sure about that. In any case, I'm interested in finding a reference for this fact.