The problem of the juror going out and collecting his own evidence (either in person or via the net) seems to be missed by some.
Any evidence collected (physical evidence, forensics, depositions, etc.), by either side in a case, requires two things:
1 - a clear chain of custody such that it can be shown unequivocally that the evidence has not been altered in any way since it was collected, or that any alterations made (a snip from a piece of material for chemical testing, say) are clearly noted, and;
2 - Full disclosure is made by both sides to each other so that each side can be prepared on the day of the trial to answer to that evidence. It is impermissible, for instance, for the police to present evidence at the trial that the defendant's attorney has not had time to prepare a defense against.
Both of these qualifications allow for the trial to proceed on as level a field as possible.
Evidence collected by a juror violates both of these requirements, since neither side has had opportunity to vet the source and relevance of the evidence, nor had opportunity to prepare to answer to it.
Further, jurors are, theoretically, expected to form no opinion on a case until all evidence has been presented (unlikely as that is in reality, it is, nonetheless, the attitude that they are supposed to be cultivating.) and collecting evidence, even if only to have further questions answered implies a violation of that criterion for a fair and impartial trial, as well as violating the evidentiary rules.
In short, it's a really, REALLY dumb move by someone who clearly thinks that he's the smartest guy in the room (and we KNOW how well THAT usually turns out!)