Sampling is very common now, and can lead to court cases where permission is not sought and appropriate royalties are not paid.
Plaigarism, even when unintentional, is also actionable - the late George Harrison was successfully sued by the writers of He's So Fine by The Chiffons, he claimed ignorance of the similarity, which is not seen as a valid excuse in law.
The 'jury is still out' on Paul Weller using The Beatles' Taxman riff for his song Start! and recently, Robin Thicke should be coughing up to the estate of Marvin Gaye for stealing Got To Give It Up so blatantly.
The list goes on - too long to list here though.