In the comments to "Character amnesia revisited", Joanne Salton remarked that "It doesn't have a tremendous effect on the ability to communicate because the odd mistake doesn't matter all that much." I started to dash off a brief reply, but my answer soon grew to such inordinate length that it seemed to merit separate posting under the above title.
If one is talking about written communication, which I assume Joanne is since she's dealing with characters, then unfortunately occasional mistakes have a huge impact on the ability to communicate. Joanne herself makes the comparison to "having every word in the English language being something that might cause a spelling mistake", and that is actually a very good analogy, though perhaps a bit extreme. But there's a great difference between mistakes in writing with Chinese characters and mistakes in writing with the Roman alphabet.
If you mess up the spelling of English words, the person who reads what you wrote can almost always make perfect sense of it. For example, you can write "it's" or "its'" when you mean "its", you can write "neccesary" or "necissery" or any of a dozen other permutations and combinations when you mean "necessary", you can write "beleive" when you mean "believe", and so on and so forth. I have correspondents who constantly make exactly the same sorts of mistakes, but it doesn't hamper our communication in the slightest, though for someone who cares about spelling, grammar, and punctuation, it can be a little bit jarring and occasionally even a little bit humorous to encounter this sort of fractured English writing. And we've all seen those experiments where the internal letters of English words are jumbled around or other sorts of transformations are worked on English words in a passage, and yet you can usually read them off without much difficulty.
The situation is very different with Chinese characters. If you forget one stroke or misplace one line or dot, or if you write one stroke in slightly the wrong direction or if you cross strokes that are just supposed to touch or that are supposed to get close but not quite touch, or if one of your strokes curves in the wrong way, etc., then your attempt to write that character will usually FAIL altogether. What frequently happens is that the writer who cannot remember exactly how to write ALL of the strokes of a given character in their proper orientation (the correct sequence is also important) will simply give up. They will either smudge, cross out, or otherwise erase the character they were trying to write, and in handwritten texts I've often seen Pinyin inserted to replace the failed character or following what remains of the crossed out attempt to write a character.
I suppose that the cold, hollow, empty, sinking feeling one experiences when one realizes that one can't remember how to write a particular character is best encapsulated in the expression "tí bǐ wàng zì" 提笔忘字, which means that you pick up your brush, pen, pencil, stylus, fingertip, or other writing instrument, but the character just won't come out. You simply have to confront the fact that you have forgotten how to write the character as a whole, though you may know one or more of its components.
Contrast that with the situation in English. You pick up your writing instrument and have the gnawing feeling that you can't remember exactly how it is spelled. If you're a perfectionist, it will grate upon you, but you can still write it out in an approximate form and be confident that your reader will understand what you meant — so long as you get the sounds roughly write (whoops! I meant "rite", no, "right"!). And if you're typing, well nowadays the spell checker will tell you you're wrong, and you can keep retyping till you get it right. And I've been astonished at how incredibly good Google is at suggesting correct spellings for words that you've mangled. Think of all the different ways you can misspell "innocuous", for example. Google seems to be able to catch all of them instantaneously. It's like a miracle to me that Google can do that. Right while I am mistyping, Google is correcting me!
Now, there's nothing like that in Chinese. If you are trying to write a character by hand or enter its shape into a computer via components or stylus / fingertip, if you don't really know for absolutely certain how to write it, the computer will not be able to help you. However, if you type in the sound of the character, the computer / cell phone / gizmo / gadget will write the character correctly for you. No more "tí bǐ wàng zì" 提笔忘字 ("lift brush forget character").
Here are a few relevant Language Log posts: