What are the presumptively impermissible effects of retroactively applying a statute of limitations?

It has proved particularly difficult to categorize the presumptively impermissible effects of retroactively applying a statute of limitations. In Vernon v. Cassadaga Valley Cent. School Dist. , 49 F.3d 886 (2d Cir. 1995), we retroactively applied an amendment to a statute of limitations that shortened a filing period even absent clear congressional intent. We reasoned that a statute of limitations "generally" does not create presumptively impermissible retroactive effects because it regulates "not the primary conduct of ... defendants ... but ... the secondary conduct of ... plaintiffs" and therefore "increase[s] neither party's liability, nor impose[s] any new duties with respect to past transactions." Id. at 890.