system
Given the following post, respond as if you're commenting on Reddit:
human
Aren't some experiments very hard or expensive, and thus not reproducible? Why isn't this a problem?

One of the tenets of science is the idea that experiments should be replicated to be verified. But some experiments are too complicated to be so. 

Giant expensive experiments like the LHC, or complicated ones like the Phoenix landers, experiments that depend on a given political will like the collecting of moon rocks, some things that are sheer luck like finding the Antikythera mechanism and maybe even some truly opporunistic experiment like the Voyagers that depended on a very precise planetary alignment to get their slignshots – those are things that are probably not being replicated, or at least not in the exact way. We sent other probes to mars and the outer solar system, but never to the exact same spot the previous ones were ("yes we don't see those Voyager bubbles but maybe they're there and not here").

So what happens when one of these experiments come up with some incredible piece of data? How can scientists assure themselves this wasn't a fluke, a experimental error or the aliens playing a prank on us?
gpt
So the trick with particle colliders, is that you generally build two or more experiments simultaneously. RHIC has STAR and PHENIX still operational, LHC has ATLAS, CMS, ALICE, LHCb all running simultaneously. That's how we "verify/reproduce" the data. 

Recall a few weeks ago, the hubbub over the mass bump in the CDF experiment at Tevatron. They claimed it was a bump in the data approaching 4-5 sigma over background. But the sister experiment at the Tevatron, D-Zero, did not find the bump, so it's likely to be improper background subtraction/error etc. 

And finally, each time we build new experiments we reproduce the old data. LHC has been "recovering the standard model" in its initial turn-on. Reproducing results that other detectors have seen before it.