If you're body's not accustomed to it, you might get some bad gas. Gradually increasing the amount of high fiber foods you eat each day might help minimize that effect.
Vegetables are good for you. Some have more carbs then others so check the carb content of what you're eating before indulging or they may effect your blood sugar levels. I've never heard of any kind of toxic effect of eating too many vegetables unless they were contaminated prior to eating them (ie recent ecoli scare with tomatos). So yeah, washing your food first is a good idea.