Well, casting doubt on knowledge assumed to be true, which may lead to overturning, but is still provisional when you're at the point where you have one published experiment claiming a result, and one published failure to replicate that result. Even correct results are not always successfully replicated on the first replication attempt, including in "hard" areas like physics/chemistry. Lots of possible reasons. One frequent case is that the original results were, even if broadly correct, underspecified, i.e. there was some key component of the original experimental apparatus or method that wasn't sufficiently described in the paper, either due to an oversight or because the original authors didn't know it actually mattered to the outcome. Sometimes figuring out what that was makes it possible to replicate the result, although other times it turns out to undermine the original results (e.g. the entire result turns out to be due to some specific impurity in a specific supplier's equipment). But in any case still very useful.
Yes! Been there, done that. So much of lab technique is hard to explain in words. People do postdocs and sabbaticals in order to learn technique. And then there are uncontrolled variables. Classic example:
> When a student had difficulty in crystallizing a compound [professors] would simply shake their beards over the flask containing the offending substance. Then, lo and behold, after nucleation had done its job, crystallization set in. Gerhard believed that the beard of an Adolf von Baeyer or an Otto Wallach could indeed be a source of crystals of every conceivable space group.
