Here's a riddle, asked and answered by a federal appeals judge.

Question: What do plea bargains and tattoos have in common?

Answer: Both are drawn in permanent ink. And they may not age well.

So said Chief Judge Edward Carnes in an opinion from the U.S. Court of Appeals for the Eleventh Circuit Monday.

Carnes reversed Chief Judge Kristi DuBose of the Southern District of Alabama in her ruling that a sentence ordered under a plea bargain could be reduced later. Carnes directed the judge to re-impose the original sentence for drug trafficking and weapons charges.

Judge Robin Rosenbaum and Judge Patrick Higginbotham of the Fifth Circuit, sitting by designation, concurred.

"In negotiating a plea bargain both sides aim for the best terms they can get, placing bets on what the future will hold," Carnes wrote in his opening. "The problem is that the future and certainty are strangers and not everyone wins a wager. Sometimes a deal, like a tattoo, does not age well and what appeared to be attractive in the past seems unattractive in the future. But plea agreements, like most tattoos, are written in permanent ink and cannot be redrawn just because one party suffers from the plea bargain form of buyer's remorse."

Carnes continued, "This case is here because two defendants convinced the district court, over the government's objections, to take up the judicial pen and redraw their freely entered plea agreements whose ink had been dry for nearly a decade."

In July 2006, Kendrick Melton entered into a plea agreement, pleading guilty to conspiracy to possess with intent to distribute cocaine and possession of a firearm during a drug-trafficking offense.

In June 2007, Glenda Flores pleaded guilty and was convicted of conspiracy to possess with intent to distribute cocaine.

While neither sentence was set by the plea agreement, both deals contained a suggestion that the length of prison time may be reduced in return for "substantial assistance" to investigators. Melton had an initial sentence of five years' imprisonment for the firearms charge and 10 years for the drug conspiracy, to run consecutively. Flores had a 10-year prison sentence. Both had been reduced to statutory minimums because of their "substantial assistance."

In 2014, base sentencing levels for a number of drug offenses were reduced. Melton and Flores moved to reduce their sentences in response. The judge approved, over the government's opposition.

"Generally speaking, a defendant who was sentenced to the mandatory minimum penalty established by statute is not entitled to any relief," Carnes said, "even if a retroactive amendment to the guidelines would otherwise reduce his sentencing range below what it was before the amendment."