In the IF community, people use the word  mimesis  to refer to the extent to which players feels like they re in the world of the story, experiencing what the player character is experiencing.   Breaking mimesis,  therefore, means reminding the player in one way or another that she s actually sitting in front of a computer typing on a keyboard and watching words scroll by on a screen.
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This is not what  mimesis  means in all contexts, and indeed the term has been used to refer to many different things over the centuries, some of them diametrically opposed to one another.  As far as I know, the term first appears in Plato s Republic to refer to speech within a play in which it is not supposed to be the playwright speaking: a soliloquy in which a character enumerates the reasons why all kings should be killed, for instance, may not reflect the views of the playwright, even though the playwright wrote every word of the speech in question.  Plato contrasted mimesis with  diegesis,  speech in which the poet actually is the speaker, though later developments in narratology have cast doubt on whether this is even possible.  Aristotle extended the term  mimesis  to include not just speech but action, which muddied things considerably; DW Lucas wrote an extended piece on mimesis which eventually concluded that Aristotle's use of the word is hard enough to define that it comes down to a matter of "Ah knows it when Ah sees it."
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Moving into relatively modern times, Henry James and Percy Lubbock used  diegesis  and  mimesis  essentially as fancy words for  telling  and  showing.   Of course, on the most literal level, a novel is 100%  told,  mediated through language.  Still, there are passages in novels which make the reader feel as though the narrator is speaking to her, and others which make the reader feel as though the action is unfolding before her eyes.  G rard Genette called the latter phenomenon  the illusion of mimesis. 
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Genette and Shlomith Rimmon-Kenan further confused the meaning of mimesis by more or less reversing Plato's meaning, using the word  diegesis  to refer to those elements of a text that relate to the world of the story and  mimesis  to refer to those elements that relate to the world of the author the world the author shares with the rest of us.  This dovetails with the way Marxist critics use  mimesis,  referring to the degree to which the world of the story reflects the world outside the story; for instance, does the story make clear that the wealth of the main character derives from the suffering of impoverished throngs in the Third World, or does it take place in a fantasy world in which capitalism has no victims?
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This leads us back to mimesis in IF.  Often, it is just this sort of divergence from the real world which reminds the reader that the world of the program is artificial.  A classic example is the implementation of a typical house in IF.  Visiting the bathroom is rarely a crucial element of an IF storyline.  Many authors would rather leave the bathroom out entirely, as the bathroom would serve merely as a distraction and would require the author to spend time and energy implementing the various fixtures therein.  But leaving out the bathroom breaks mimesis, because the player ends up wondering, what kind of house doesn t have a bathroom?  Inventory limits are another example.  Should the player character really be allowed to haul around thirty-eight different items, half of which would reasonably require both arms to carry?
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The other main means by which mimesis is broken has less to do with oddities in the world of the story and more to do with the means by which it is delivered.  After a player has tried half a dozen ways to phrase a command and had them all rejected, she is unlikely to feel like she s inhabiting the world of the story anymore.  Similarly, a piece of IF that tries to strike a dark, somber tone probably ought not to offer the default lighthearted quips as error messages, for they will undoubtedly ruin the mood of the piece.  Some works even begin with an atmospheric introduction and then immediately yank the player out of the world of the story by chirping,  Type ABOUT to learn about this game! 
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Players differ on the amount of mimesis they expect or even want a piece of IF to provide.  Most are very happy not to have to worry about how long it has been since the player character has eaten or used the toilet, even if these are often pressing concerns in real life.  On the other hand, few will take kindly to blatantly unrealistic elements like a front door of a suburban house that can only be unlocked by manipulating the garden gnomes on the lawn; it s just too divorced from reality.  But no matter which part of the spectrum you aim for, you won t be able to please everyone.  Best to choose the level of mimesis that suits your particular project and roll your eyes when players quibble about the presence or absence of bathrooms.