I can post two discouraging but probably very general events, I could not recall any specific event that was discouraging. (My apologize for my rather late post.)
The first one is about the general “that is an easy to solve problem” comment. I think the most discouraging comment to me that I had many times over the course of my academic path (and I still encounter it very often) is the comment that the solution to a problem I stated should be easy. I found it very discouraging when I hear that something should be supposedly “very easy” but I have not solved the problem after hours. I think there are two motivations behind this comment 1) people tend to speak “lightly” about things they have sometimes not really a clue about, 2) they don’t have any idea but want to encourage you to try, 3) they know what they are talking about but don’t know what your level is. My experience tell me that the first is usually more common. The second and third have a good intention but can be harmful.
As a solution, I started asking to what other problem they compare whether the particular problem is easy or not and try to make a realistic estimation for myself.
The second is about a grant proposal that I had submitted but was given the comment by someone in the committee that I will not get it. Although
there was no particular reason that the person had to tell me this, he gave me this information during one of our meetings. Afterwards I had to give a presentation to a core committee and was really nervous because I had this comment all the time in my head. At the end, I got the grant after my presentation but to this date I do not know what this person meant by this comment.
The learning I think is not pay attention what others say and not to discourage myself by others comments, which may not be even intentionally stated. (Although not easy.)