Under Rule 11, the party seeking sanctions must serve the motion on the opposing party and give that party 21 days to withdraw the offending pleading. Here, the defendants apparently served the sanctions motion, but they failed to file it before the plaintiffs’ complaint was already dismissed by the district court. The district court refused to hear the motion. The court explained its ruling in this passage:
Courts have uniformly denied Rule 11 motions where, as here, a motion was not filed until after the case was dismissed. See Roth v. Green, 466 F.3d 1179, 1193 (10th Cir. 2006) (motions for Rule 11 sanctions filed after district court dismissed complaint should have been denied; citing cases from four other circuits); VanDanacker, 109 F.Supp.2d at 1054.
The courts have reasoned that to grant Rule 11 sanctions based on a motion filed post-dismissal would defeat the purpose of the safe harbor provision. The advisory committee notes to Rule 11 provide strong support for this conclusion:
Ordinarily the motion should be served promptly after the inappropriate paper is filed, and, if delayed too long, may be viewed as untimely. . . . Given the “safe harbor” provisions . . . [in Rule 11(c)(2)], a party cannot delay serving its Rule 11 motion until conclusion of the case (or judicial rejection of the offending contention).Fed. R. Civ. P. 11, Advisory Committee Notes (1993 Amendments). A leading federal practice treatise explains the rationale for this rule: “if the court disposes of the offending contention within the 21-day safe-harbor period after service, it becomes impossible under the provision of Rule 11(c)(2) to file the motion or otherwise present it to the court.” 2 James Wm. Moore et al., Moore’s Federal Practice § 11.22[1][c] (3d ed. 2014); see also 5A Charles Alan Wright and Arthur R. Miller, Federal Practice and Procedure § 1337.2, at 727 (3d ed. 2004) (“[S]ervice of a sanctions motion after the district court has dismissed the claim or entered judgment prevents giving effect to the safe harbor provision or the policies and procedural protections it provides, and it will be rejected.”).
Because defendants’ motion for Rule 11 sanctions was not filed until after the Court dismissed plaintiff’s complaint with prejudice, the motion is untimely and must be denied because it defeats the purpose of the safe harbor provision ofRule 11.[3]
It makes perfect sense that the court did not want to waste further time on a case it had already dismissed. To do so, would be a waste of judicial resources.
Edward X. Clinton, Jr.
Source: Noonan v. CACH, LLC, Dist. Court, ED Missouri 2016 – Google Scholar
fedcivilprocedure 
Do you have a link to that order? It doesn’t make any sense. It conflates “service” and “filing.” If a Rule 11 motion is served, and the 21 day period passes, then the serving party files the motion after dismissal, the purpose of the safe harbor has been fully vindicated. And that process also makes sense. If a defendant serves a Rule 11 motion, but the plaintiff ultimately prevails, or even survives a motion to dismiss or for summary judgment, then the Defendant won’t ever file the Rule 11 motion. In Roth v. Green the Defendant did NOT serve the Rule 11 motion on the Plaintiff until he actually filed it so the Plaintiff never had the benefit of the safe harbor. The 10th Circuit’s language was sloppy in referring to the “filing” of the motion after dismissal when the problem in Roth v. Green was the failure to even SERVE the motion until after dismissal, and it appears to have led to this Court making a major mistake. Even the cases cited by the district court here talk about failure to SERVE the motion prior to dismissal. And in reality parties FILE Rule 11 motions after dismissal all the time, and if they have complied with the 21-days-earlier SERVICE requirement they are often granted.
OK, found the link and read the order. The Court got the right result because the Defendants did not comply with the 21 day safe harbor, but the dicta about the motion being improper if filed after dismissal is completely wrong. Every case cited involved a situation where the moving party had not complied with the 21 day safe harbor service requirement, or where the court dismissed the case during the safe harbor period. The language about the safe harbor functioning as a “practical time limit” to FILE a Rule 11 motion makes absolutely no sense–you CAN’T file the motion until after the safe harbor has expired.