Not knowing / being ignorant of a rule. It's your job to read the rules, and it's our job to enforce them.
Getting banned for something that someone else did. Just because they did it, and didn't get caught, doesn't mean you won't. It's also not a valid defense, a dispute is about you, don't bring another party into this, use the report button instead and serve out your ban.
Open a dispute for:
Cruel / unusual ban durations.
Accidental bans that were done for no reason.
Staff abusing or targeting you.
Something that might be against the rules and you were banned, but had a very valid reason for it. Note: this can also be determined to be a pointless dispute, so be careful when you post it.
Escalating a dispute to an administrator.
Clarification on a ban reason.
Lying to a staff member in a staff dispute is a permanent ban. If you lie, your dispute is automatically void, and a limited duration ban may be extended to permanent, along with your alternate account.