Though mostly rare, this does happen occasionally. Legally, a therapist cannot approach or acknowledge a client first, as this would be considered a breach of confidentiality. Additionally, there can be unintended consequences, such as, for instance, if a person is in therapy but their family does not know, it puts the client in an awkward position to try to explain who this person is that said hello to them. Clients can approach or say hello to their therapist outside of the therapy room if they wish! A therapist just cannot be the first to acknowledge the other.