I was checking around in the task area, and one of my units was invalid. Can I get some documentation or explanation of what has to happen for something to go invalid?

If I should guess in this case (after looking at the stderr), than your result in not similar enough to the two other results, the only strange thing is, that it should be "Completed, marked as invalid" and not "Validate error".

de_separation_14_2s_05_3_test_1_1340149896_1126908_0:
<background_integral> 0.000610335136917 </background_integral>
<stream_integral> 491.560243093927400 239.201611002556940 </stream_integral>
<background_likelihood> -6.577453494973065 </background_likelihood>
<stream_only_likelihood> -8.080880174792412 -5.741850440842178 </stream_only_likelihood>
<search_likelihood> -4.439295200379255 </search_likelihood>

de_separation_14_2s_05_3_test_1_1340149896_1126908_1 (yours):
<background_integral> 0.000610726237968 </background_integral>
<stream_integral> 491.768742249492500 239.298347775637350 </stream_integral>
<background_likelihood> -6.577731700537138 </background_likelihood>
<stream_only_likelihood> -8.081064345167638 -5.742025529796699 </stream_only_likelihood>
<search_likelihood> -4.439484153478863 </search_likelihood>

de_separation_14_2s_05_3_test_1_1340149896_1126908_2:
<background_integral> 0.000610335136917 </background_integral>
<stream_integral> 491.560243093928818 239.201611002557627 </stream_integral>
<background_likelihood> -6.577453494973067 </background_likelihood>
<stream_only_likelihood> -8.080880174792414 -5.741850440842181 </stream_only_likelihood>
<search_likelihood> -4.439295200379257 </search_likelihood>

I think the colors make it pretty clear, how different your result is compared with the two others. As long as it's just one, I wouldn't worry too much about it.
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Are results 0 and 2 suppose to be the same as well? Could we have gotten a bad workunit? Also, are Invalid's rare but normal if you get one?

The occasional invalid/error is to be expected. We all get them once in a while. It's nothing to worry about.

Alright, I was worried that I was starting to break my BOINC...