There are various reasons this error might come up. In my case I ended up with this error when i tried to add an item in to a list in a site which has Anonymous access enabled.
I just got an error “Unexpected error occured” & the browser stayed in the newform.aspx page. But when I checked the list new item is being created in the list.
So there is no issue with the permissions. And I came to know that “Anonymous users cannot start a workflow or an event handler“.
Then I noticed there was a feature which I have written for a custom list. I deactivated that and it worked well..
PS : But still there must be a way to start a workflow/event handler by an anonymous user. If any one find so, Please post.