John Cunningham writes:
Bennett v. Lally, C.A. 9545-VCN (September 5, 2014) is a recent Court of Chancery case about actions that can result in a person’s being a fiduciary of another person without even knowing it. It’s very relevant to lawyers who give “informal advice” to people. This advice could create a lawyer-client relationship. ... You can read the opinion here.
It's not just lawyer-client relationships, of course. As I explain in my book Agency, Partnerships & LLCs:
An agency relationship comes into existence when there is a manifestation by the principal of consent that the agent act on his behalf and subject to his control, and the agent consents to so act. The requisite manifestation of consent can be implied from the circumstances, which makes it possible for the parties to have formed a legally effective agency relationship without realizing they had done so. The purpose of the relationship need not be a business one; in theory, if you send a friend to the vending machine to get you a soda, you have retained an agent. ...
The requisite consent may exist even where the parties are unaware that their relationship constitutes an agency relationship and did not intend for their relationship to carry with it the legal consequences of creating an agency relationship. To be sure, there is no such thing as an “unwitting agent,” in the sense that every agency relationship requires knowing consent by both parties. What then is it to which the parties must “consent”? The principal must consent that the agent shall act on the principal’s behalf and subject to the principal’s control. The agent must consent to so act. If they do so, they have an agency relationship, even if they did not “consent” to the legal consequences that follow.
 Restatement of Agency (Second) §1.
 See, e.g., A. Gay Jenson Farms Co. v. Cargill, 309 N.W.2d 285, 290 (Minn. 1981) (“An agreement may result in the creation of an agency relationship although the parties did not call it an agency and did not intend the legal consequences of the relation to follow.”).
 State v. Luster, 295 S.E.2d 421 (N.C. 1982) (“We find the phrase ‘unwitting agent’ to be a contradiction in terms.... An agency relationship must be created by mutual agreement.”).