switch between tu and usted

In lesson 2.4 of Spanish both speakers switch constantly between tu and usted form, for instance Maurice says:  mira(tu), siga(usted), entiendes(tu). Amy also switches all the time. That sounds highly unrealistic. Please explain.

I didn't listen to the lesson, just read the transcript. It looks like in the first two cases, mira and siga, Mauricio is using the second person singular imperative form, giving her an instruction. Then he asks her if she understands: ¿entiendes? which is the second person indicative, just asking her a question.

I could not find any examples in the transcript of using the usted form.

It was interesting to me that when I was in Costa Rica, I frequently heard people using the usted form, and learned that it is a sign of the politeness and deference natural to Costa Ricans. It seems like speakers from other countries use it less often.

Dan-H24, thanks for you reply. However:
1) siga is usted form of imperative, the tu-form is sigue (the verb is seguir)
2) Vaya una cuadra is also usted-form, the tu-form is ve
Only -ar verbs end on -a in the tu-form of imperative, otherwise on-e.

Later Amy says: 'Repite por favor'. Since the verb is repitir, she seems to have switched to the tu-form, otherwise she would have said: repita por favor.
 

There are mistakes in the course: some I find surprising (coming from a native speaker), others may be intentional (so as not to overwhelm with the subjunctive tense).

I can only imagine two contexts where you would see mixing "tu" and "usted" in a conversation: 1) one person is "testing the waters" to see if the other wants to move to a more intimate relationship; and 2) when tempers flare (either to be insulting or to reflect a temporary distance in the relationship).

I just went back and looked at lesson 2.4. Yeah, that is weird. I can actually imagine though a conversation like that happening between a native and non-native speaker where the dynamic is very different than between two native speakers.
 

My experience here in Peru is that there are times when the formal and informal are mixed during everyday conversation.  I'm not sure why.  It's not always out of anger/frustration or "testing the waters" either, though that has been the case at times.  I really am not sure why it happens in other cases, but it does.  Let me add that these conversations are usually in an informal situation such as lunch at a restaurant.  Also, after a informal conversation, many times the people in the church will use the formal usted form when saying God bless you.  It's really quite rare in the church I'm helping with to hear "Que Dios te bendiga" even when the whole conversation up to that point had been in the informal. 

the-hefay - Could you ask the people you are in touch with about this? I get the formulas (e.g., God bless you) but mixing "tu" and "usted" between native speakers sounds really strange to me.

One addition to my previous post. There is also the case where someone may say something like "You know when you're young that..." and they will use the "tu" form even though it's an "usted" relationship. The "you" is generic in this case (i.e., not referring to the person in front of you).
 

Steven, I'll ask a couple of people and see what they say.  In addition let me add that although it happens here in Peru, it's not an everyday common place occurrence.  However, it does happen.

Thanks. I'll look forward to your follow-up. You must be getting quite fluent now being down there in Peru.