How should I categorize a transaction that I am disputing with my bank/credit card?

You can categorize both the original charge and the reversal of the charge (when your dispute is successful and the bank/credit card company reverses the charge) as "Other Miscellaneous Expense".

Note that if it is a checking/savings account transaction the reversal would come through as a Cash Inflow since it is money going back into your account.

If you have already categorized the erroneous transaction to another account, it’s not a problem, just categorize the reversal to the same account.