Essentially, I wonder that if X is a plural noun, should we use a plural or singular verb in a sentence such as "X are a subject that [verb] ..."? I believe that we should always use the singular verb form there (for example, "Eatable flowers are a subject that comes up ..."). However, I was surprised to find this sentence on the Eatable Flowers FAQ page:
Eatable flowers are a subject that come up in almost all cooking classes eventually and I am a big fan of them.
as well as this on Google Books:
Tools are a subject that come up at our house from time to time.
I think they are typos, but I would like a double check to be sure.