Is it incorrect not to use "sia"?

Question

A sentence like

Li eniris sian domon

uses the word sian to refer to the li-person. If one writes

Li eniris lian domon

can lian then refer to the li-person? Or will it technically refer to another person?

Answer 1

The use of sia is mandatory, there are no exceptions.*

Every competent speaker will understand the sentence

Li eniris lian domon

as referring to two different men.

*There are cases of complex constructions involving more than one acting person, where there can be hesitation whether sia or another pronoun of the third person is more clear. This, however, does not contest the validity of the rule. Cf. the concerning chapter in PMEG.

Answer 2

I look at it this way. "Si" and "li" are different pronouns. Using the wrong pronoun in Esperanto is just as wrong as using the wrong pronoun in another language.

In English you have to use "her" if the person you're talking about is female. If I were to say "Mary went into his house", everybody would know I was talking about someone else's house.

In Esperanto, we have a pronoun which is used to refer to third-person subjects. If we use a different pronoun (like lia), and said something like Marko eniris lian domon everybody would understand this to mean that we were talking about some other person.