Although the KISS Approach does not focus on errors, the psycholinguistic model explains why some errors deserve to be called such, and thus encourages students to avoid them.  In this sense, an "error" is anything that hinders the processing going on in the readers' short-term memory. Miller suggests that the normal STM has seven slots, and I suggest that we use only some of these slots for processing sentences. In the process of reading, the reader must also have in mind at least the writer's main idea (thesis), the purpose of the paragraph (topic sentence). Some of the "slots," I suggest, are used for keeping track of these. The problem with an "error" is that it may force the reader to focus on a specific bit of text, thereby using extra slots of STM to process a sentence. As it does so, it may have to discard the main idea of the paper, the purpose of the paragraph, etc.
     We should be honest with students and note that an occasional error, depending on the context, may not be important. I use the analogy of running on a rock beach. In doing so, most people can keep their balance after an occasional trip. But if two, three, four trips occur in relatively rapid succsion, the runner will fall on his fanny. Of course the terrain (context) is also important. The looser the pebble, the rougher the terrain, the more likely the runner is to fall after only a single trip. Similarly, most readers can easily recover from a single error, but if the sentences are complicated, they may not. And, if a short paper is peppered with errors, the reader is almost certain to crash. When that happens, the reader loses track of the writer's ideas. It is the writer who suffers, especially when the writer is a student doing a paper in history, plastics, etc. Their instructors expect the students to have taken English (It is, after all, required.) and thus to have learned how to write sentences. Their instructors are not going to take the time to re-read the paper to get themselves back on track and re-find the writer's ideas. 
     The model suggests, in other words, that an "error" is anything that would cause a reader to use more "slots" than necessary to process a sentence. Most of these errors fall into three categories.

Spelling Errors

     The model, in effect, slows down a process (reading) that happens very quickly. The model also suggests that good readers do not simply react to the text, they learn to anticipate what is coming. When a reader hits the word "to," he or she expects to find an answer to the question "To what?" When a reader hits the word "too," no such question exists. Thus a misspelling confuses the reader. In "She wanted to go too the store," the reader will have processed "too" as meaning "also" and then hits "the store," which doesn't have anything to chunk to. The reader thus needs extra slots to figure out what to do with "the store." Similar problems result from the misspelling of "its/it's," "they're/there/their," and most of the other horrible homonyms that give many students problems. The model, however, shows that these "errors" are not just rules made up by English teachers. They are errors because they throw any reader off track.

Fragments, Comma-Splices, and Run-ons

      Because the KISS Approach focuses on how sentences work, it does not need to pay special, individual attention to fragments, comma-splices, and run-ons. The model, however, does explain why these are serious errors -- they interfere with the normal processing suggested in the model. With a fragment, a reader finds an "ending" punctuation mark, a signal to dump to long-term memory, but what is currently in short-term memory does not include a full S/V/C main clause pattern. As a result, the reader may have to figure out for him or herself what the fragment meaningfully "goes with." To do so, the reader must use additional slots in STM.
     Comma-splices and run-ons cause a different kind of error in that two main clauses are connected with no signal to the reader to dump the first one to LTM. The reader therefore attempts to chunk elements of the second main clause to the first, but it doesn't work. Here is an interesting example from the writing of a seventh grader:

He coached a team and I played on it, [when I was eight] we won all our games.

Misplaced (Dangling) Modifiers

     The principal of chunking, and the emphasis on meaning, helps many students see, and learn to handle, problems with misplaced modifiers. A college student wrote:

Being of an impulsive nature, my mother often accompanies me when purchasing clothing. 

Errors and Style

      Eliminating errors such as those discussed above is probably the most important thing a writer can do to improve his or her style. The other aspects of syntax and style all concern making one's writing appear more mature, more logical, more graceful. Eliminating errors, on the other hand, eliminates confusion, and the last thing one wants to do as a writer is to confuse the reader.Simple and correct is always better than complex and confusing.