Copyright is not the only legal aspect which must be considered when you want to use the content of a work. There are other statutory, contractual or ethical regulations which can play a role irrespective of whether a work is protected by copyright or not and whether the possible uses of a work are permitted pursuant to the Copyright Act. This applies, in particular, to the right of personality, the right to privacy, the prohibition of unfair competition and the plagiarism ban.
For example, a paparazzi photo of a celebrity that the photographer may not have been allowed to take due to the infringement of the celebrity’s personality rights; or a scientific work which is no longer protected by copyright is used in its entirety without citing the source – from a copyright perspective, it may be used; however, without citing the source, it amounts to plagiarism, i.e. it breaches good scientific practice.