In my writing, I observe the distinction between that and which, using that for restrictive modifying clauses and [comma] which for nonrestrictive clauses. I try to teach my students the difference, but I do not make it a topic of intense focus in the first-year legal-writing course.

When you see mistakes, the mistake is almost always using which without a comma. Thus, the reader is not entirely sure if you intended a restrictive clause but misused which or a nonrestrictive clause but neglected the comma. Like this:

The lawnmower which is broken is in the garage.

This could mean—

The lawnmower that is broken is in the garage.

• I have more than one lawnmower, and the broken lawnmower is in the garage.

or

The lawnmower, which is broken, is in the garage.

• I have one lawnmower. It is in the garage. By the way, it is broken.

Are you still with me?

Well, yesterday I read this sentence. The writer used [comma] that for a nonrestrictive clause—something you almost never see:

• Agent Diaz said he engaged in a conversation with Mrs. Hanover through a window at her apartment and delivered, to her husband, a copy of the subpoena, that  Mr. Hanover placed on the kitchen table.

Perhaps of interest only to writing nerds.