Federal Rule of Evidence 407 prevents a party from introducing "subsequent remedial measures" to prove that a party was negligent (and a few others). The purpose behind this rule is to encourage people to fix problems with their behavior/products to prevent harm and injury without fearing that an adverse party will point it out at court to show that they had acted negligently.
However, adverse parties may use that evidence to prove other things, such as ownership, control, impeachment, etc.
A recent Fifth Circuit Court of Appeals addressed another issue that arises with this rule: what is an earlier "injury or harm?" What if the remedial measure is merely improving the product?
Novick v. Shipcom Wireless, Inc., 946 F.3d 735, 739 (5th Cir. 2020)