There is a legal concept called the cumulative effect of errors. One description of this concept is: “In some cases, the cumulation [sic] of minor errors may amount to error requiring [a decision by a judge], even if individual errors, alone, would not.” (Via LexisNexis. Subscription required.)
You have probably noticed an analogous effect in your reading. If an author keeps making errors (or keeps using awkward diction or syntax), eventually you will conclude that he is careless and probably unreliable, even if none of his errors reduces clarity by much.
A recent article provides a good example of cumulative effect. Here are the first 200 words of the article, with my questions and comments interspersed:
On Friday, news broke that CNET had been forced by its parent company CBS to remove the Dish Network's Hopper set-top box from its “Best of CES” awards due to ongoing litigation between the two companies.
[Does “the two companies” refer to CNET and Dish Network? Or CBS and Dish Network? Spell it out so I don’t have to ask.]
CBS has been battling the Dish Network in court
over the Hopper's ability to skip past commercials automatically (NBC, ABC, and Fox are also taking action).
[Wait – what’s this? And what is its relationship with CBS and with CNET?]
representatives told The Verge that the Hopper with Sling
[How is that different from the Hopper you mentioned earlier? Is it different?]
had been “withdrawn from consideration” from the “Best of CES” awards due to CBS's lawsuit with Dish; that the ban on coverage is limited only to specific products implicated in ongoing litigation with CNET's parent company;
[Instead of “CNET’s parent company,” just say “CBS,” for clarity.]
and that the ban only applied to product reviews and that news coverage would be exempt. That policy appears to have been hastily put in place. Prior to the move Friday, CNET had reviewed the Hopper and written extensively about the device.
But the problems may go deeper than that. The Verge has now learned that the facts of the case are somewhat different than the story CNET and CBS had previously shared
[I’m still waiting for you to tell me who “CBS Interactive” is.]
with the public. According to sources familiar with the matter, the Hopper was not simply an entrant in... (200 words)
[Is that the Hopper with Sling or the plain old Hopper?]
The Takeaway: As you edit your copy, watch out for the cumulative effect of errors. The more errors you make, the worse you look, even if none of your errors reduces clarity by much. Eventually your reader stops reading, at which point the effective clarity of your text drops to zero.