The odd thing is, comma placement

Question

“The odd thing is, she has no idea who he is and why she got all of his heritage.”

I would normally identify this first sentence as having a nominal that-clause acting as a subject complement with the “that” omitted. Which is fine, but it would mean it's incorrect to use a comma in that position, but I get the hunch, the inkling that it is correct even while the sentence below cannot take a comma no matter what. Am I just plain wrong? Would there be any other real grammatical reason for there to be a difference other than emphasis?

“The odd thing is that she has no idea who he is and why she got all of his heritage.”

Answer 1

With the comma, the speaker is making a comment about the statement she has no idea... This is similar to using "hopefully" as a sentence adverb as in Hopefully, he'll come early. Without the comma we have Odd thing = She has no idea that ..., which is more of a statement equating something to being odd. I see a difference in emphasis: I find it odd that... or *Isn't it odd that...?" versus It's odd that... With the comma I feel the presence of a speaker; without, it has a more narrative feel.

There is an analogy with cleft sentences, which emphasize a later part of a standard-order sentence by bringing it forward (You ate my cookies! -> Those were my cookies you ate!), except here the emphasis is achieved by "detaching" the odd, which is already at the beginning. Perhaps pointing out the oddness was a recent decision.

Answer 2

In your example, the comma is used for clarity when the that is omitted.

Without the that and absent the comma, you effect a garden-path sentence, where (in this case) the noun following the linking verb wants to be a nominal subject complement before it gets around to being part of a zero that-clause.

For illustration purposes, I will use a different sentence as an example:

The odd thing is, this relationship may actually work.
_{Source: The Washington Post — Between Kid Quarterback and Coach, Something Unexpected Has Come to Pass}

Let’s look at this three ways:

The odd thing is that this relationship may actually work.

The odd thing is this relationship may actually work.

The odd thing is, this relationship may actually work.

The second, garden-path version leads to The odd thing is this relationship. — leaving the reader to get back on the trail for the rest of the sentence.

Here are some examples from the Corpus of Contemporary American English. You can see that the comma-less, that-less uses are not very common.

the odd thing is that

the odd thing is ,

the odd thing is NOUN

the odd thing is the

Circling back to your own example . . .

Believe it or not, the The odd thing is she. is a pretty good set up for a garden path, using — as it “properly” does — the nominative pronoun in the predicate.