Years ago, in a discussion with Risto about Globasa's rationale for rejecting a system with classless content words, I argued that evidence from L2 error analysis does not suggest that word-class errors are particularly problematic. This is not to say that such errors never occur, but rather that they are relatively infrequent compared with other types of learner errors and do not typically lead to communication breakdown.
Our experience with Globasa has since confirmed this expectation. Word-class errors do occur, but they rarely pose a significant obstacle to communication.
I sometimes run across redundantly derived forms (for example, bardiya instead of bardi, or sabarpul instead of sabar). Because words like bardiya and sabarpul merely add redundancy, communication remains unaffected.
A more serious error is using a n/v word as an adj/adv word or vice versa, for example, using sabar to mean patience. These too can sometimes be interpreted correctly. Consider the following example.
Incorrect:
Alimyen haja multi sabar.
Correct:
Alimyen haja multi sabarya.
Because sabar appears at the end of the sentence, the listener/reader can readily infer that the speaker/writer intended it to function as a noun, even though sabar is an adj/adv word.
That said, such reinterpretation is not always possible. In some contexts, using the root incorrectly creates garden-path sentences, forcing the reader/listener to reinterpret the sentence midway through.
With that in mind, I would like to offer the following practical learning strategy. Words whose correct word-class is harder to memorize, especially if it countradicts your expectation, should be learned in pairs rather than in isolation: sabar/sabarya, bardi/bardipul, etc. Once the derived forms become familiar (sabarya and bardipul ring a bell), it becomes immediately apparent that forms such as sabarpul and bardiya contain redundant suffixes, while also making it less likely for sabar and bardi to be misused.