"Such unbounded discretion carries with it grave potential for abuse." Atwater v. City of Lago Vista, 532 U.S. at 372. (O'Connor, J., dissenting).
This case concerned the arrest of Gail Atwater, a woman who, unfortunately, forgot to buckle up her children who were riding in the front seat of her truck. Atwater was pulled over by a Texas cop who yelled at her and berated her. When she asked the cop to lower his voice because he was scaring the children, he responded by jabbing his finger in her face and saying, "You’re going to jail." He then handcuffed her, placed her in his squad car, drove her to the local police station, took away her possessions, and locked her up. The children would have been taken into custody too, but for the intervention of a friend of Atwater who happened to be passing by.
The constitutional issue in Atwater was whether police officers may reasonably make a full-blown custodial arrest EVEN when the offense is a minor vehicular or administrative violation carrying no jail or prison time and EVEN when there is no threat to the public safety. By the margin of a single vote, the Supreme Court said yes and Americans became a little bit less free.
To the Atwater majority, probable cause equals reasonableness, end of discussion. To the four dissenters, a full custodial arrest is such a severe intrusion on an individual’s liberty that reasonableness requires an inquiry as to whether the arrest was needed to validate legitimate governmental interests.
What was most irritating about Atwater is that it bushed aside as an imaginary "a parade of horribles" the obvious real-world implications of its decision in order to adhere to a bright-line rule. (Everybody loves a bright-line rule, it is too bad that messy humanity eventually turns them all into a mockery of themselves.) If a particular police officer stops you for a minor offense, it is his or her call as to whether you end up with a warning, a citation, or whether you end up undergoing the shock of incarceration. And is there any doubt that the officer's discretion will be exercised against a person whose race or gender or youth or antiwar bumper sticker trigger his or her negative feelings?