One of the many commentaries the
Midrash shares about the giving of the
Torah explains that the
Angels in
heaven protested
God's handing over of the
Torah, the
divine composition upon which the heavens and the
earth were
founded, to a
creature as
lowly as a man. The Angels requested instead that they themselves should receive the
Torah.
Among the many answers the Midrash brings (God asks Moshe (aka Moses) to answer the Angels himself), is God's answer: The one chance you had to uphold the Torah you failed. You see, some Angels were sent to the physical world, to visit Avraham (aka Abraham) to comfort him after his circumcision. There Avraham served them a meal composed of both meat and milk.
The Torah prohibits the cooking of milk and meat together as well as eating food prepared in that fashion. While there is disagreement in the Midrash as to precisely which prohibition the Angels violated, (One opinion explained that Angels are made of the elements Water and Fire, and so when they consumed the milk and then the meat, it cooked within them.) everyone agrees that they messed up their one chance and that paved the way for mankind to recieve the Torah. (woohoo!)
R' Nachman of Breslov used this as the basis for his teaching on the incomprehensible nature of free will. He taught that just as we cannot grasp how it is that we are able to choose freely, the Angels cannot understand how it is that they do not have free will. You see, the heavens and the earth were all created in order that man would be, and man was created in order to receive the Torah. So, the Angels, part of the creation of the heavens themselves, couldn't understand that their existence was part of a course of events that necesitated that which they were protesting.
As to whether the Angels had a choice about eating the milk and meat, the answer could go either way. Most likely the Angels did not have free will and this experience was simply an irrefutable example that they weren't fit to receive the Torah. Alternately, if one maintained that Angels temporarily clothed in physical form gain a measure of free will, then there are Mephorshim (aka comentators) who explain the Angels' complaint like so: The Torah is made up of a group of letters arranged in a particular order. In the higher spiritual realms, the order of and spacing between letters is interchangeable. The Angels thought that they could receive the Torah in some alternate ordering of the letters, something which would be appropriate for Angels. In such a Torah there would be no prohibition against eating milk and meat. When God refused their request, it became clear to the Angels that God meant to bestow the Torah ordered the way it was.
sources: Midrash Rabbah, and the teachings of Rebbe Nachman in Likkutei Halacha as taught to me by R'Peretz Ohrbach of meah shearim