As far as I can see, there are no significant arguments against the principle that all events have a cause, which is to say the principle of sufficient reason. (It's important to note that the seemingly identical idea that all effects have causes is a circular argument based on the mutual definitions of "cause" and "effect".) While the idea seems intuitively obvious and therefore self-evident, we hold many counter-intuitive ideas to be true.
Has anyone proposed a serious argument that events sometimes are not caused?
Clarification: The question title may be misleading because it suggests that the question is an epistemological one, but my actual question is metaphysical (or perhaps even ontological). Whether or not we can always (or even ever) know the sufficient reasons for an event is beside the point (unless it can be shown that we always can know the cause of every event).
I've been asked to define what I mean by an event. That's a bit more than I can take on at the moment, but the Stanford Encyclopedia of Philosophy suggests that we have a "prima facie commitment to entities of this sort." If I had to suggest a definition, I'd say an event is
a discrete observation or inference about a period of time. That I was married is an event that was observed by many people. That the sun was formed is an event inferred by the current state of the universe. Of course, that definition has an assumption buried in it that makes the question less interesting: inference implies causation. So we need to find a definition that conforms to our intuition of what an event is, but does not implicitly conform to our intuition that events are caused.
For the purposes of this question, the best definition of an event is that it is something that happens. Do things happen for which there is no cause?