Also according to the federal rules of civil procedure, subsequent remedial actions cannot be used as evidence. Essentially this means that if the defendant takes action to fix the problem, the plaintiff cannot bring this action into court as evidence.
The example given in my civil law class was this. A tiger jumps over a fence at a zoo and kills a guest. The next day the zoo makes the fence taller. Family of the dead guest sues the zoo. They cannot use the fact that the zoo built the fence taller as evidence that the zoo was negligent by originally building a shorter fence.
The reasoning behind this is that it would make situations more dangerous because defendants would be afraid to fix things, as this fixing could prove that they were negligent in the first place. In the zoo example, the zoo would not want to build the fence taller for fear of opening themselves up to liability. This makes it more likely that someone would be killed again. In the target example, target might be afraid to make sure the balls were safe for fear of making themselves liable for the issue, possibly resulting in more accidents.
It's strange and confusing, but that's just how the law is most of the time.
EDIT: Rule 407, explanation for anyone interested:
Rule 407. Subsequent Remedial Measures