I get it. You author an opinion in a tax dispute and think that the IRS acted too late, so find in favor of the taxpayer. One of your Ninth Circuit colleagues dissents, but you prevail because the district judge that is sitting by designation from Vermont is on your side. But even though it's just a tax dispute, the court takes the case en banc, and despite the fact that you're drawn for the en banc panel (yay!), you lose (boo!).
So, understandably, you file a dissent from the en banc opinion, which in you argue your side, with the same arguments that you already made in your now-vacated panel opinion. But you also up the ante a bit, telling the world that the en banc opinion will totally screw things up and is supremely wrong on the merits. You start your dissent with the following line: "Today, our court throws our tax system into disarray." And then get into more detail on just how wrong you think today's decision is. Claiming that the en banc opinion is not only "perplexing" but also conflicts with both "the Tax Code and common sense," and that the correct resolution of the case is "straightforward" and not at all complicated.
All that rhetoric is perhaps understandable. But I must say that it's a little strange coming from an en banc decision in which Judge Bumatay's opinion on the matter gets outvoted 10-1, without a single judge on the en banc panel -- even those judges who typically vote alongside Judge Bumatay -- adopting his position in this one.
Sometimes you're totally convinced you're right even though the rest of the world, including some very bright people, says you're wrong. Other times, a bit more modesty is perhaps warranted.