For instance the transport of goods between areas of grossly different prevailing price for a good or different tax levels. It needs firm police intelligence and keeping ahead of the incredibly clever scams such as carousel scams for VAT fraud.
The simpler our tax regimes and the more they relate to real physical entities the less room there is for confusion in which dark things can occur.
Muddled tax regimes are fruitful for crime. For instance the Carbon Credit trading system seems to lay itself open to such abuse because it is complex and to do with abstract concepts.