Actually, I was watching Lord of the Rings and there´s this passage where Smeagol is trying to get the ring back and I found it quite interesting when he said:

"They're thieves! They're thieves! They're filthy little thieves! Where is it?Where is it? They stole it from us, our precious. Curse them! We hates them! It's ours it is, and we wants it! We wants it, we needs it. Must have the precious. They stole it from us. Sneaky little hobbitses. Wicked, tricksy, false!"

Of course, In real life things may be different but with that character it could not be any different. Maybe we should compare their - it´s the rules - to our: são as regra; … quando eu estava subindo as escada...; já fechou as porta? Neither of theses sentences are right but they’re not unnatural either.

I find that when it comes to plural of words in spoken language, people in general don't care that much.