Well,"对不住" doesn't mean u really did something wrong,however,u might disappointed someone or he/she wasn't satisfied with u.More importantly,such phrase is often used among acquaintances rather than strangers.And people usually say this phrase to others instead of saying it to the person u disappointed directly(hope u know what i mean:P).And I don't think it's the most normal way to say sorry,though "对不起","对不住" and "不好意思" can all translated into "I‘m sorry".But generally speaking,people use "对不起" and "不好意思" more. Maybe my answer can help. :)

"对不住"is almost the same with"对不起",but there is a little difference between them. we'd rather use"对不起" to strangers,but if you use "对不住"to a stranger,it's a little bit rude,and maybe they will think you are kidding,don't think you are truely wrong. "对不起"means 'it was my fault.' "对不住"not only means 'i'm wrong.' it also means ‘i was thoughtless on this thing and didn't take good care of you.’ But In general,there is not too much difference between them.

一般我会对比较亲密的人朋友说。比如：对不住了兄弟。

when ur relationship is very good. and its sth not important things troubled ur friend. then u can say that