1. Manually inputting incorrect data into an enterprise system.
2. Saving a file with corporate financial or tax information onto a personal device
3. Accidentally deleting a custom excel formula used to calculate corporate tax data
4. Override data in an enterprise system and replace with data / information calculated outside of the system.
5. Working on a non-secure public WiFi network.
Source: The top tax & accounting mistakes that cost companies millions, Bloomberg BNA, Software products December 2014