A common and recognized mistake resulting in legal malpractice is the double use of an index number.  in Wilk v Lewis & Lewis, P.C. ;2010 NY Slip Op 05897 ;Decided on July 2, 2010 ;Appellate Division, Fourth Department  the index number came from an earl er pre-complaint discovery motion.  It might also come in a motion for leave to file a late notice of claim or some other variant.

Attorneys got the discovery but then sued under the same index number.  At the time, such a filing was a nullity, which had statute of limitations significance. 

"Defendants commenced a pre-action discovery proceeding against plaintiff’s employer to obtain information concerning the accident and, when defendants thereafter commenced a Labor Law and common-law negligence action on behalf of plaintiffs (hereafter, underlying action), they used the same index number that had been used in the pre-action discovery proceeding. Supreme Court granted the motions of the defendants in the underlying action (Labor Law defendants) to dismiss the complaint. Under the law at that time, the failure to purchase a new index number rendered the action a nullity because it was never properly commenced (see Chiacchia & Fleming v Guerra, 309 AD2d 1213, 1214, lv denied 2 NY3d 704). No appeal was taken by plaintiffs from that order, although plaintiffs retained other attorneys (plaintiffs’ successor counsel) shortly prior to the expiration of the time in which to take an appeal. Plaintiffs commenced a second Labor Law and common-law negligence action against the Labor Law defendants, who moved to dismiss the complaint as time-barred. We previously reversed an order denying those motions and instead granted the motions and dismissed the [*2]complaint (Wilk v Genesee & Wyoming R.R. Co., 45 AD3d 1274). We concluded that the second action did not relate back to the filing of the underlying action pursuant to CPLR 205 (a) because the failure to purchase a new index number rendered the underlying action a nullity (id. at 1275).

Defendants also failed to establish that plaintiffs could not prove the remaining elements of a legal malpractice cause of action. Defendants contend that their negligence was not a proximate cause of plaintiffs’ injuries because plaintiffs’ successor counsel did not file a notice of appeal when the Court of Appeals issued its decision in Harris v Niagara Falls Bd. of Educ. (6 NY3d 155). We reject that contention. Defendants are correct that the Court of Appeals changed the law by holding in Harris that a defendant could waive a defect in connection with filing requirements such as the failure to purchase a new index number (see id. at 159). Even assuming, arguendo, however, that we agree with defendants that the time within which plaintiffs could file a notice of appeal expired 35 days after the final Labor Law defendant had served the order dismissing the first complaint against the Labor Law defendants (see Blank v Schafrann, 206 AD2d 771, 773; Williams v Forbes, 157 AD2d 837, 838-839; Dobess Realty Corp. v City of New York, 79 AD2d 348, 352, appeal dismissed 53 NY2d 1054, 54 NY2d 754), we note that the time in which to file a notice of appeal against that final Labor Law defendant expired [*3]approximately 30 hours after the Harris decision was issued. It cannot be said that the failure of plaintiffs’ successor counsel to learn of the Harris decision and file a notice of appeal within that narrow time period constituted an "intervening and superseding failure of plaintiff[s’] successor [counsel]" to file a timely notice of appeal (Pyne v Block & Assoc., 305 AD2d 213). Defendants thus failed to establish that "plaintiff[s’] successor counsel had sufficient time and opportunity to adequately protect plaintiff[s’] rights" (Somma v Dansker & Aspromonte Assoc., 44 AD3d 376, 377; cf. Ramcharan v Pariser, 20 AD3d 556, 557; Albin v Pearson, 289 AD2d 272). "