Immersion is the key value of RPG world. Immersion breaks are therefore most important flaws in the game design.
Definition: Immersion is a state(similar to Flow) where player feels included in the fictional world and his actions are organic in the setting. Immersion breaks when player feels the artificial nature of the world and that his actions in-game are artificial or forced.

The model of immersion breaking is as follow:
1. Trigger: Event that forces the player to reevaluate in-game relationship with his pre-conceived notion of "how the game should run".
2. Response: Player is forced to abandon the fantasy-state of "suspension of disbelief" and correct his behavior/actions to fit with new situation.
3. Doubt: The set and setting of the world come clashing with new interaction player just experienced, forced the player to adjust his expectations and notions of the expected game laws.
4. Disgust or Acceptance: Player either begins to feel disgust with new revelation or accept it as "acceptable break from reality", readjusting his expectation of world.

Types of triggers:
A.Game content that feels out of place(such as blatant advertising or cash shop promotions). or ill-designed to the genre(acrobatic puzzles during a serious quest).
Generally, that content is a distraction to in-world setting, removing which doesn't change anything else.

B.Forced UI interaction: UI notifications,unexpected or unnecessary dialogs, masking in-world content or disrupting combat with non-combat reasons(inventory management, potions without hotkeys).
Combat loop interruptions are probably the most serious immersion-breaking flaw.

C.In-world content mismatch with setting(inorganic content): a flaw that only becomes apparent when comparing overall design with specific area.
Examples: Dinosaurs and dragons in same setting. Firearms in medieval magic fantasy. Unexpected technology in magic setting in general(unless intentional or sci-fi-based). General breaks with single-player experience(e.g. non-instanced quests where people complete the same thing multiple times).

D.Non-Intuitive game mechanic: a mechanic that requires external references or preparation to use/exploit, but is required for proper gameplay. Typically the mechanic feels like an artificial limitation or concept that clashes with the fictional realism of the game. Example: resistances to element X can't go past 50%(due balance) and getting past that requires "resistance cap raising" modifiers on items.

E.Unexpected new additions to a mechanic: player notions of specific mechanic(example: trading items to others is an action solely between two players) are broken, when he discovers that e,g, tax of 1% must be paid on each trade. This mechanic isn't apparent at first or creates a sense of unfairness/artificial addition(such as economic gold-sink) that discourages the use of the mechanic(due to e.g. unexpected cost/time investment).