This is really the central question in legal malpractice.  While the popular conception is that one needs to find the mistakes, the truth is that there are mistakes in every human endeavor.  What counts in legal malpractice is the linkage to a bad outcome.  However, Justice Schaffer gives a much better response in Landau, P.C. v LaRossa, Mitchell & Ross ;  2010 NY Slip Op 50620(U)
Decided on April 7, 2010 ;  Supreme Court, New York County ;  Schlesinger, J.
 

"This brings the Court to its final discussion of what precisely the plaintiff has to prove to recover from the defendant, and whether either side is deserving of having its motion granted.

As everyone here acknowledges, because it is the law (!), a plaintiff in a legal malpractice case, must do more than simply allege and prove that his attorneys were negligent in representing him; he also must show that the negligence made a difference in the outcome, so that a plaintiff must show that he would have prevailed or received a greater award but for the negligence of his lawyer. [See, e.g., Rudolf v. Shayne, Dachs, Stanisci, Corker & Sauer, 8 NY3d 438 (2007)] Or in the case of a defendant, would not have suffered the damages he did. The latter situation is what the plaintiff argues here. He urges that because there was ample evidence to show the cases would have likely settled or been successfully tried without the tainted evidence, even if those awards might have been less than was actually received, the damages that he ultimately paid back to the City would have been less, and perhaps much less. The opposing sides differ as to how much the plaintiff has to prove on that point at this stage of the proceedings.

In this regard, a legal malpractice action has often been described as a trial within a trial. This is so because the plaintiff must adduce evidence that shows at least a likelihood that he would prevail on the underlying merits of the case. So, for example, in the First Department decision, Tanel v. Kreitzer & Vogelman, 293 AD2d 420 (2002) the Appellate Division found that the defendants’ motion for summary judgment in the malpractice action should have been granted. While it was conceded that the defendant law firm was negligent in failing to move for a default judgment within one year of the hospital’s default, the plaintiff had failed to successfully oppose the affidavit of the medical expert who opined that there was no medical malpractice. Therefore, the plaintiff was unable to show the underlying case had merit or that but for the negligence of the [*6]defendant, they would have prevailed. All that was required there, as suggested by the Court, was to successfully present competing proof to show that their claim had merit. But they did not and so the action was dismissed.

But here, the plaintiff does show, despite the contrary arguments made by defense counsel, that in both actions there was testimony and documents to prove a prima facie case on the issue of constructive notice, the only seriously contested issue, without the use of the tainted evidence. The 1995 reargument papers by the defendant, for which they must take responsibility, prove that point.

So has the plaintiff here done enough to satisfy the "but for" element of a legal malpractice case? I believe it has. As the Court said in Gladden v. Logan, 28 AD2d 1116 (1st Dep’t, 1967):

Obviously it is impossible to show conclusively that had that action gone to trial it would have resulted in a verdict favorable to plaintiff. What plaintiff must do is to prove such facts in regard to the accident as enables a jury to find that she would have recovered.

As was said in Zarin v. Reid & Priest, 184 AD2d 385 (1st Dep’t. 1992 citing to Carmel v. Lunnay, 70 NY2d 169). "The test is whether a proper defense would have altered the result of the prior action.’ "The plaintiff maintains that it is likely here that it would. The defendants insists that it would not have. Thus, I find a classic dispute regarding the interpretation to be given to the same facts. This, then, should result in a denial of both motions, and it does.

Eisen, P.C. should finally have its day in court on its claim that the defense did not adequately oppose the City’s motion leading to a settlement in the latter’s favor for over a million dollars. There were factual contentions made by the City in that motion that were never challenged. That arguably was not simply a matter of judgment. I say that, because nowhere in the voluminous papers submitted by the defense, is the argument ever made that there were good reasons or any reasons for the omission. The only explanation proferred was that it was Mr. Eisen’s idea and decision to exclude that argument. For reasons previously discussed, that is not good enough.

The plaintiff is not entitled to summary judgment on liability because it is part of its burden in a legal malpractice to show both aspects, i.e., that there was substandard representation which influenced the result in an unfavorable way. The defendants counter that they were not negligent, or that even if they were, there would have been no different result. "