I've been posting on (a large open source php/MySQL project) message board about scaling the software to millions of users and millions pageviews a day. On one thread I offer a total of 6 changes, then a single user:

First, celebrates my changes because the software is known to have problems at that scale,
Then, asks for clarification because he's unable to understand a simple change in the first changeset,
Then misunderstands the point of the second changeset and wrongly advises another user to delete the entire block instead of work around it with a conditional as I instructed, and
Finally, tells me that adding a single index (MySQL) to a single integer column on a table will be prohibitively expensive and that it's better to suffer through a few full table scans than add a single index.

I don't understand how an exchange like this even happens. He's already demonstrated he doesn't understand this software, doesn't understand scale, and probably doesn't program very well in general, why does he think he can jump up and correct someone who's offering fixes to the community that they've verified in production? Forum posters confuse me.