Banks commonly apply hidden interest fees using a practice called "Residual Interest" (aka "Trailing Interest") to mislead their Credit Card holders.
For example, if you have a $1000 credit card bill (at AMEX, for instance) and you pay $950 at the end of your billing cycle, AMEX will charge you interest on the full $1000, rather than only on the $50 remaining balance. They argue that the practice is "perfectly legal" and that "trailing interest" allows them to arbitrary revoke payment grace periods.
This behavior has been widely criticized by a broad range of consumer protection groups as it is counterintuitive, deceptive, and sadly, Congressionally-approved common practice.
Please move to end these anti-consumer, deceptive practices by the Banks.