The 15-month sentence handed down to Dennis Hastert last week put some surprise into a plea bargaining system that doesn’t like surprises. When the former US House Speaker pleaded guilty prosecutor agreed to a sentence of zero to six months.

The judge more than doubled that.

A former federal prosecutor says that kind of unpredictability can make it hard for defendant to analyze the risk of going to trial verses taking a plea. But Hastert’s case was an aberration in so many ways that his surprisingly long sentence shouldn’t affect other plea deals.