Published June 2, 2018 Updated March 14, 2021
Circumcision opponents often exaggerate the risk of complications in order to terrify parents considering the procedure. Intactivists warn parents that their son could end up like the boy whose penis was destroyed in a botched circumcision. Raised as a girl, he committed suicide at age 38. An exploration into the full story of what happened is more complicated.
Electric cautery machine malfunction
Identical twins Bruce and Brian Reimer were born in Winnipeg, Manitoba in 1965. At 6 months both boys had difficulty urinating, and they were diagnosed with phimosis. They were scheduled for hospital circumcisions.
A doctor who did not usually perform circumcisions was assigned to the Reimer twins. Rather than use one of the traditional circumcision methods, she used an electric cautery machine with a sharp needle. However the equipment malfunctioned, sending a surge in current that burned off Bruce’s penis. Consequently the operation for Brian was canceled. 
Transition from Bruce to Brenda
The parents were referred to Dr. John Money, a psychologist at Johns Hopkins Medical Center in Baltimore, Maryland. Money argued that gender was a social construct. Specializing in sex changes, he recommended sex reassignment surgery so that Bruce could be raised as a girl. At 17 months, Bruce became Brenda, and was castrated one month before his second birthday. He was not told that he had been born male. 
Money published a paper describing the experiment as a success. Time magazine uncritically reported, “The experiment has apparently succeeded. Aided by plastic surgery and reared as a daughter, the once normal baby boy has grown into a nine-year-old child who is psychologically, at least, a girl. This dramatic case, cited by Medical Psychologist John Money ... provides strong support for a major contention of women's liberationists: that conventional patterns of masculine and feminine behavior can be altered.” 
As the author of a book about David’s life  explained, the reality was quite different.
At age 2, Brenda angrily tore off her dresses. She refused to play with dolls and would beat up her brother and seize his toy cars and guns. In school, she was relentlessly teased for her masculine gait, tastes, and behaviors. She complained to her parents and teachers that she felt like a boy; the adults—on Dr. Money's strict orders of secrecy—insisted that she was only going through a phase. Meanwhile, Brenda's guilt-ridden mother attempted suicide; her father lapsed into mute alcoholism; the neglected Brian eventually descended into drug use, [petty] crime, and clinical depression. 
The Reimer twins had annual psychological checkups with Money. During these visits, they were subjected to practices that later were recognized as unethical and abusive. As an adult David characterized the sessions with Money as "torturous and abusive." The twins
were directed to inspect one another’s genitals and engage in behavior resembling sexual intercourse. Reimer claimed that much of Money’s treatment involved the forced reenactment of sexual positions and motions with his brother. In some exercises, the brothers rehearsed missionary positions with thrusting motions, which Money justified as the rehearsal of healthy childhood sexual exploration... Reimer stated that Money observed those exercises both alone and with as many as six colleagues. Reimer recounted anger and verbal abuse from Money if he or his brother resisted orders, in contrast to the calm and scientific demeanor Money presented to their parents. 
Transition from Brenda to David
At age 14 Brenda threatened to commit suicide if she were forced to see Money again. Her parents explained that Brenda had been born a boy. Shortly thereafter the teenager decided to become David and reassert his identity as a male. At age 15 David had reconstructive genital surgery. Later he got a job working in a slaughterhouse. At age 25 he fell in love with a woman, married her, and adopted her three daughters. Through reconstructive surgery, David was able to have sexual intercourse and experience orgasms. 
Still, David continued to be “plagued by shaming memories of the frightening annual visits to Money, who used pictures of naked adults to ‘reinforce’ Brenda's gender identity and who pressed her to have further surgery on her ‘vagina.’” 
If that weren’t enough, David suffered a series of disasters as an adult. The downward spiral began in 2002 when Brian, who was suffering from schizophrenia, died from an overdose of anti-depressant drugs. David visited his brother's graveside daily. He lost his job and fell deep into debt.  On May 2, 2004 his wife informed him that she wanted a separation. Two days later, at the age of 38, David Reimer killed himself with a shotgun in a grocery store parking lot. 
An honest appraisal
Intactivists cite the David Reimer story as a “worst case scenario” – not to educate, but to instill fear and terror in the minds of parents who plan to circumcise their newborn sons. But the circumstances surrounding the botched circumcision wouldn’t apply to most newborn circumcisions. Most importantly the electro-cautery method that burned off his penis is rarely if ever used today. Our resident physician had never heard of it used for infant circumcisions.
Most circumcision in the U.S. and Canada are performed on newborns. But David was 6 months old when he and his brother were diagnosed with phimosis. This was not an elective circumcision.
Furthermore, a number of other factors likely played contributing roles in his decision to end his life: 
the decision to raise him as a girl
the abusive "treatments" to which the twins were subjected
the bullying that he suffered throughout his childhood
the depression and drug abuse that plagued family members
the suicide of his twin brother Brian
his financial debt and long-term unemployment
his wife's request for a separation two days prior
An honest appraisal of David's life would accept that several other setbacks helped to bring him to the point of suicide.
The David Reimer story is a cautionary lesson about the dangers of trying to force a change in sexual identity. Circumcision opponents who frighten parents by telling them their son could end up like David are simply fear-mongering. Newborn circumcision today is extremely safe when performed by a trained professional in a modern medical facility.  
 Elaine Woo; "David Reimer, 38; After Botched Surgery, He Was Raised as a Girl in Gender Experiment"; Los Angeles Times; May 13, 2004
 Phil Gaetano; "David Reimer and John Money Gender Reassignment Controversy: The John/Joan Case"'; The Embryo Project Encyclopedia; November 15, 2017
 “The Sexes: Biological Imperatives”; Time; January 8, 1973. Time erroneous identified Reimer as 9 years old; at the time of publication he was 7.
 John Colapinto; “As Nature Made Him: The Boy Who Was Raised As a Girl”; Deckle Edge; 2000
 John Colapinto; “What were the real reasons behind David Reimer's suicide?”; Slate; June 3, 2004
 American Academy of Pediatrics: “Task Force on Circumcision Technical Report”; Pediatrics; September 2012. “Significant acute complications are rare, occurring in approximately 1 in 500 newborn male circumcisions … The majority of severe or even catastrophic injuries are so infrequent as to be reported as case reports.”
 World Health Organization; “Manual for early infant male circumcision under local anesthesia” p 7; 2010. When male circumcision is performed by well-trained, adequately equipped and experienced health-care personnel, these complications are minor and rare, occurring in 1 of every 250 to 500 cases. Most of the complications can be easily and rapidly addressed and do not result in significant morbidity or mortality.”