Need help using bracketing commas with parenthetical elements

Okay, this is something I'm really confused about:

We use bracketing commas to set off an interruption to add extra information. Here is an example:

Television today, some might say, is diabolic.

The phrase "some might say" is just extra information. However, I have read on websites that when a parenthetical element follows a coordinating conjunction (and, or, but, etc) used to connect two independent clauses, a comma is not put in front of the parenthetical element. Example:

"The Red Sox were leading the league at the end of May, but of course, they always do well in the spring." (no comma after "but")

However, in a book written by Stephen King, here is what it says:

"He also tried very hard to do great works, but, unfortunately, he didn't succeed so well at that."

In this sentence, it appears that Stephen King has used a comma after the coordinating conjunction "but." Now let me write the sentence again with no comma after "but."

"He also tried very hard to do great works, but unfortunately, he didn't succeed so well at that."

Now there is no comma after the word "but." Which one is right, or are they both right? I'm so confused about this. Any advice would be much appreciated.