The first thing you will notice is probably the "Check Engine" light. Flex fuel is an Oxygenate, which means it actually has oxygen molecules in it. This will make the engine run lean, causing a P0171 and/or P0174 fault code (assuming its an OBD II vehicle). Once the tank is run dry and regular fuel put back in, this will clear itself up.

Other effects is possibly a clogged fuel filter, since "Flex Fuel" is 85% alcohol and will usually remove varnish buildup from inside the tank and lines, which may clog the fuel filter. may want to replace it (if you have one) once the tank runs out.

Long term effects (of prolonged use) is degredation of the fuel system. Alcohol is corrosive and Flex Fuel systems use special O-rings and other components to resist this type of corrosion.

Long story short, run the tank out, fill up with regular gas and don't do it again! Hope this helps.