A HostileInsertion is when someone (often a HostileStudent) writes a ridiculous one-line challenge in the middle of a complicated screen-long argument, destroying its stylistic integrity and internal coherence.  (That said, it ought to be noted that not all such insertions are done with hostile intent.  In particular, for an ''unsigned'' contribution, it may be appropriate).

Your intellectual juices are flowing, but get interrupted by someone demanding an answer to some basic question. Usually the answer is completely self-evident, or the question is meaningless, but requires a very lengthy elaboration.

Hostile insertions make you wonder whether you're dealing with a complete neophyte to WikiWiki or a deliberate attack. Either way, it's exceedingly tempting to just wipe them out and be done with it.  A better solution is this: move the insertion and any necessary context to below the original content, so they no longer disrupt the flow of the original argument.  

''Maybe I spend too much time on http://fark.com, but the name of this page does not put pleasant thoughts in my head.  UFIA, anyone?''
* Yes, you spend too much time there. Mind in the gutter! Never even occurred to me. There is no problem with the name of this page. This is back to the giggling at "sperm whale" mentioned recently. Let it go. (Not being a fark regular, I was unfamiliar with "UFIA" and so I foolishly googled it. I am not amused, in this context; that's way more than I wanted to know.)

See MigrateInlineComment.
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See also DontLookAtTheFinger