This violation may not be obvious to you if you are not a fuel seller or a firefighter. The UN number* shown in the red diamond is 1203 which indicates that the storage tank is holding gasoline. The danger sing indicates that the tank is holding diesel fuel. Gasoline is a flammable liquid that has a very low flash point (ignites at -45 degrees). Diesel is a combustible liquid that has a high flash point (ignites at 100-130 degrees depending on the grade). 

This is very dangerous as the way a fire department might approach an incident involving this tank would be dramatically different for gasoline vs. diesel.

* UN Numbers are four-digit numbers that identify hazardous materials, and articles (such as explosives, flammable liquids, oxidizing solids, toxic liquids, etc.) in the framework of international transport. There is no UN number allocated to non-hazardous substances.