Edited: I rewrote the answer completely after reading the other question.
From the way you write this question, I guess that your intent was to say “The last example you labeled as ‘valid’ is not actually valid.” However, in the context of the other question, I would not understand the sentence “The last valid example is not” that way at all. (I do not know whether it is said to be “grammatically correct” or not, but that is irrelevant.)
When I see the sentence “The last valid example is not,” I somehow expect that something which appeared before that sentence is elided after the word “not.” Probably the word “valid” in the same sentence is too close to the location of the ellipsis.
In the current case, the conversation went like this:
You know, is it an error or not? – Jon Purdy
I think the last valid example is not. – Claudiu
I would interpret the last sentence as “I think the last valid example is not an error,” and I would never imagine that anything else might be intended.