Non-reflexive and reflexive pronouns are usually introduced in a manner that suggests the distribution is complementary. (In the following examples, I use indices i, j, ... to indicate coreference; an asterisk * indicates coreference is not possible.)
Juliaj sagt, Annai denkt nur an sichi, *j.
Juliaj sagt, Annai denkt nur an siej, *i.
However, this is an oversimplification. For instance, as the Duden grammar (8th edition) points out (§ 367), there are contexts in which both a reflexive and a non-reflexive pronoun are possible.
Annai sah die Hundej auf sichi/siei zurennen.
Moritzi erwartete mich bei sichi/ihmi im Büro.
In the absence of other criteria, people will always cite the potential for ambiguity as a reason to avoid one construction over another. However, language is easily capable of tolerating a certain level of ambiguity.
Annai sah die Hundej in ihremi, j Garten rennen.
Moritzi erwartete Hansj in seinemi, j Büro.