A list of common roommate clashes occurring in both real life and fiction:
Dramatic differences in standards of cleanliness (typically someone either being filthy or messy, or having ridiculously and unrealistically high standards of cleanliness and tidiness), or habitual failure to do household chores (or do them properly).
Perennial lateness with and/or failure to pay rent or bills, and often also an unwillingness to contribute to communal supplies and/or deliberately purchasing the cheapest, lowest-quality supplies they can find when they do contribute.
Dysfunctional communication styles, usually either conflating aggression, belligerence, and hostility with directness, or by communicating in an overly indirect or oblique manner, then growing passive-aggressive when they feel that no one is listening to them.
Unauthorized consumption/usage or theft of food, beverages, and other household goods.
Dramatic differences in noise creation and tolerance, typically either being extremely loud and inconsiderate (especially late at night, or involving sexual activity), or demanding unreasonably minimal noise.
Clashes over guests, typically friends or significant others who are constantly over (especially if they regularly stay over), revolving doors of hookups, or unauthorized parties.