Is there any dangers associated with taking too many vitamins and nutritional supplements?

Yes.: Moderation in all things. Do not take excessive amounts of anything. Vit d intake should be limited to no more than 2000 iu/day in the absence of some overriding concern.

Mega dose: Our body will get vitamins needed with a normal balanced diet. Exception are B12 in vegetarians, Folic Acid in pregnant women, vitamin d and calcium in postmenopausal women, and men over 60 year old. No other indication for vitamins i can recall. Going beyond the recommended dose by your physician while do not add to your health they might damage your bothy. Toxicity of vitamins can happen.

Yes but uncommon: Too much of anything, including water & oxygen, can be dangerous, but one has to take extremely high doses of supplements for them to be dangerous. Vitamin a toxicity may occur when taking 50, 000 units a day for 2 years or more but some can take 1 million units a day for 5 years without problems. To get vit d toxicity one must take over 10, 000 units/day. B6 may be toxic at doses above 250 mg/day.