There are at least three sorts of cases:

1. Learning about an algorithm (from, say, Wikipedia) but taking no runnable code from elsewhere.
2. @elasolova's cases where the handful of ways of writing something are somewhat widely known or at least widely discussed. (Everyone's Sieve of Eratosthenes looks quite a bit like everyone else's.) Background knowledge -- or what should be beackground knowledge in a given domain -- never needs to be sourced.
3. Copying an entire solution without sourcing it.

3's that aren't 2's get you yelled at on stackoverflow, expelled from school, or sued, but there's no simple rule distinguishing 3's from 2's. What counts as common knowledge is up to a given community. I have a couple times considered posting links when I found this kind of bald plagiarism, but this community's norms -- if they exist at all -- are not as clear as, say, SO's.