Is this sentence grammatically correct or punny (or both)?

Solution 1:

It is an example of ellipsis. You are eliding the word valid at the end of the sentence “I think the last valid example is not valid.”

It is perfectly grammatical. It might be considered a kind of use–mention distinction pun if you had put the word valid in quotation marks, as in

I think the last “valid” example is not.

Here, the first instance of the word valid is a mention but the elided second instance is a use. That is, you weren’t saying the example was valid, but just using the word valid as a label that someone else used for the example. In the second clause, you use ellipsis to refer back to the word valid but this time you mean to use the word valid for its meaning and not as a label.

This is definitely wordplay entailing multiple meanings, which is what a pun is, but usually a pun uses two different meanings of a word; whereas in this case, you are punning use and mention of a word.

Solution 2:

It's grammatically correct, and a little humorous, but not a pun: a pun is generally a play on the sound or meaning of a word or phrase. What you wrote is effectively the same as saying:

I think the last "valid" example is in fact not.

Note the quotes (referring to the original post) and the use of a term such as "in fact" (showing contrast), both of which can be omitted because it's clear from context, and it's wittier to be terse.

Solution 3:

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.