I've found this on the web. It might help someone who writes with the correct preposition, as most of the Americans do not know the difference between IN and ON.

Consider the professor's desk and all the prepositional phrases we can use while talking about it.
You can sit BEFORE the desk (or IN FRONT of the desk). The professor can sit ON the desk (when he's being informal) or BEHIND the desk, and then his feet are UNDER the desk or BENEATH the desk. He can stand BESIDE the desk (meaning NEXT TO the desk), BEFORE the desk, BETWEEN the desk and you, or even ON the desk (if he's really strange). If he's clumsy, he can bump INTO the desk or try to walk THROUGH the desk (and stuff would fall OFF the desk). Passing his hands OVER the desk or resting his elbows UPON the desk, he often looks ACROSS the desk and speaks OF the desk or CONCERNING the desk as if there were nothing else LIKE the desk. Because he thinks of nothing EXCEPT the desk, sometimes you wonder ABOUT the desk, what's IN the desk, what he paid FOR the desk, and if he could live WITHOUT the desk. You can walk TOWARD the desk, TO the desk, AROUND the desk, BY the desk, and even PAST the desk while he sits AT the desk or leans AGAINST the desk.

All of this happens, of course, in time: DURING the class, BEFORE the class, UNTIL the class, THROUGHOUT the class, AFTER the class, etc. And the professor can sit there in a bad mood [another adverbial construction].

What has this got to do with the confusion between 'on' and 'in'?

I don't think many would confuse 'on a desk' and 'in a desk'. The meanings of these are very clear. The problem comes with examples like 'on a field' and 'in a field'. Both of these are correct.

I think you need to allow some leniency into your rulings for 'in' and 'on'.

'The cat sat in the chair' and 'The cat sat on a chair' are both perfectly fine grammatically. They have different connotations though as they offer a different picture about the size and shape of the chair or even the size of the cat. It's a complicated subject. Most native English speakers will choose the correct one instinctively.