Liverpool were dumped out of the FA Cup by West Brom in a match littered with Video Assistant Referee referrals which caused confusion on the pitch and in the stands.
Three major incidents in the first half of the Reds' 3-2 defeat required the intervention of technology -- including the first time a match referee has used a pitchside television replay.
And although it eventually led to the correct decisions being made, the time it took -- with almost four minutes elapsing between Mohamed Salah being fouled and Craig Pawson awarding a penalty which Roberto Firmino missed -- left supporters and players on all sides angry and frustrated.
It detracted from an impressive performance by the Baggies, who became the first team since Real Madrid in October 2014 to score three in the first half at Anfield.
Jay Rodriguez scored twice in the first 11 minutes, following Firmino's opener, with Joel Matip's own goal giving the visitors a two-goal cushion.
Salah's first FA Cup goal, and his 25th of the season, set up a thrilling finale but the Baggies held on to reach the fifth round for the third time in the last four seasons.
Firmino's clever sixth-minute chip from Salah's rebounded shot after a misunderstanding between goalkeeper Ben Foster and Jonny Evans was cancelled out just 71 seconds later when Rodriguez fired into into the top corner after Georginio Wijnaldum lost possession in midfield.
Rodriguez converted again, this time from six yards, after Chris Brunt sliced through Liverpool's midfield.
The manic start was compounded when Craig Dawson headed in Brunt's 20th-minute corner, only for VAR to spot Gareth Barry in an offside position in front of goalkeeper Simon Mignolet.
Three minutes later Salah was pulled back by Jake Livermore and -- having not been satisfied with the information he received in his earpiece -- Pawson made his way to the tunnel to view the television screen.
Three minutes and 54 seconds after the incident occurred the referee returned to the penalty area and pointed to the spot but, after Gareth Barry was booked for protesting, Firmino rattled the crossbar.
West Brom losing Kieran Gibbs and Hal Robson-Kanu to injury in quick succession was only a minor sub-plot as despite all the goals, delays, confusion and substitutions Pawson played only four minutes of added time.
During that period Dawson broke free on the right and his cross was touched in by Matip at the far post but even that required a second look by VAR to ensure Krychowiak was not offside.
Early in the second half Pawson made a quick check to video referral to confirm the offside flag to deny Dawson another header was correct -- but there was no such requirement just past the hour when Foster poked a hand on the ball before connecting with substitute Danny Ings.
Ings and fellow substitute James Milner both had chances palmed away by Foster before Salah reduced the deficit, becoming the club's third-fastest player to 25 goals (32 matches).
However, Liverpool's 19-game unbeaten home record ended as they lost back-to-back matches for the first time in exactly 12 months, which was also an FA Cup exit to Championship side Wolves.
Manager Jurgen Klopp has now failed to get his side past the fourth round in three attempts.
The last time West Brom beat Liverpool in the FA Cup they went on to win it in 1968 and while that dream may still be a way off a fourth match unbeaten was an important confidence booster for the Premier League's next-bottom side.