Okay, let's see if I can explain this. A restrictive clause is essential to the meaning of the sentence and is neither preceded nor followed by a comma; a nonrestrictive clause can be omitted from a sentence without fundamentally changing the meaning of the sentence and is both preceded and followed by a comma (unless of course it ends the sentence). Sticklers such as myself never use "which" restrictively. Consider the different flavors of the following: (1) "The Best Show podcast, which bookem_dan-o prepares, celebrated its first anniversary last week." (2) "The Best Show podcast that bookem_dan-o prepares is vastly superior to the one I put together, which does not exist." The thing to remember is that if a clause can be dropped without losing the information the sentence is meant to communicate, it is nonrestrictive, surrounded by commas, and introduced by "which." If the clause is vital to the meaning of the sentence, use "that" and no commas.
When you're the writer, you know what's restrictive and what isn't. When you're editing someone else's work, things can get tricky. For example, in "The Best Show podcast, which bookem_dan-o prepares, celebrated its first anniversary last week," the main message is that the podcast is a year old, and the fact that bookem_dan-o is responsible for it is secondary. If the sentence read instead, "The Best Show podcast that bookem_dan-o prepares celebrated its first anniversary last week," it would mean that another Best Show podcast, one not prepared by bookem_dan-o did not celebrate its first anniversary. If I came across ""The Best Show podcast which bookem_dan-o prepares celebrated its first anniversary last week," I would not be sure whether the second meaning was intended or the writer had simply omitted the all-important commas.
To complicate matters further, "which" may be used restrictively (this is more common in England). Thus, when you use "which" instead of "that," emily, you're not really making a mistake; you're just not following a usage that many people prefer. If I can tell that a writer is using "which" deliberately, as a matter of style, I will leave it or at least ask before changing it. In straightforward writing, however, I will always insist on the "that"/"which" distinction, because doing so makes for less ambiguity.
More than you bargained for?