One of my favorite new GMAT students in NYC asked me to take a look at a set of verbal questions that have been floating around the internet for awhile. The file is called "Brutal SCs", and it contains 70 GMAT sentence correction questions. They're supposed to be brutal, I guess. And they are... just not in the way that the author (or compiler?) apparently intended. I'd be lying if I told you that I analyzed every single question in the document with extreme care, but I did take a reasonably thorough survey of the first 25 sentence correction questions. I found major typos in four questions, and a fifth question had a misuse of the phrase "due to" in the non-underlined portion of the original sentence. These mistakes may or may not have impacted the answer, but they definitely made me question the quality of the resource.
More importantly, several of the answers seemed to be just plain wrong. At least two questions (#21 and #22, if you happen to have a copy) are very clearly flawed, and I'm deeply suspicious of several others. In short, at least 20% of the questions in this resource contain mistakes of some sort, and I suspect that this figure is an understatement.
The bottom line: stay away from Brutal SCs, and find another way to practice your GMAT sentence correction skills.