When is a ____ not a _____?

I was having a hard time coming up with this old joke. It was on the tip of my tongue but I just wasn't getting it, and I knew that it was so obvious I would be embarrassed when I finally remembered it. Rather than asking a person for help, I asked the Google.

"When is a ____ not a _____?"*

Very often, as it turns out.

The one I had been trying to think of was: When is a door not a door? (When it is ajar)

But there were others working that same idea: When is a car not a car? (When it turns into a driveway)

And then, there was the bounty of the interweb spreading forth before me:

When is a crisp not a crisp? (When it is more like a cake or biscuit, according to Pringle attempting to get out of UK taxes that apply to the potato chip category but not other food categories)

When is a volunteer not a volunteer? (It isn't just about not getting paid)

When is a swastika not a swastika? (When it's encoded in unicode. Duh.)

When is a pegleg not a pegleg? (When you need a doctor's note to avoid having to check it on EasyJet.)

When Is a Tetrathiolate Not a Tetrathiolate (Um, yeah.)

To this list I add my own:

When is a house not a house? (When it is afire)

When is a boat not a boat? (When it is afloat)

When is a blog post no longer clever and entertaining? (OK. See you tomorrow.)

*In Google-speak, this is "When is a * not a *?".