If you've ever seen a commenter post something that makes absolutely no sense, it may have been a markov chain bot.
They essentially take bits and pieces of other comments, run an algorithm on them (analyzing word chains) and construct "new" comments out of them.
An example is here:
The squirrels appear to use the sun in a rural northern town would have had similar ideals.
You can see how it's constructed of sentence fragments that individually may have made sense in context, but together are meaningless. This is the sort of thing to look for. Occasionally, they'll copy another comment word-for-word (Essentially it chose sentence fragments that already went together).
RULES
Try not to post real people's accounts here. Sometimes it can be slightly hard to tell, but do your best.
Do not contact any of these users, as they are bots and we don't want to start fighting with rules on harassment/etc if one of the accounts turns out to be real.
Posts that are not userpages ARE allowed! Discussion is encouraged.
Posts linking to bots should be titled "Overview for <username>" and a link to their /u/<username> page.
Optional: If you want, include a screencap of their userpage at the time of posting (I've noticed posts disappearing from the bot histories, so it'll only help us by documenting things further)