Dyche stunned by Everton's late collapse against Bournemouth
Heading into the 87th minute, Everton led through goals from Michael Keane and Dominic Calvert-Lewin.
But Antoine Semenyo scored with what was then only Bournemouth's second shot on target in the 87th minute, sparking an embarrassing Everton collapse.
Stoppage-time goals from Lewis Cook and Luis Sinisterra completed a turnaround that Bournemouth boss Andoni Iraola admitted his side did not deserve.
Never in Dyche's long managerial career with Watford, Burnley and Everton had he experienced just a devastating meltdown.
"I've had a few defeats down my years as a player, coach, manager, but this is the most frustrating because to dominate a game for so long and come out of it without something at least is incredibly frustrating," he said.
"These are ones that as a manager you scratch your head at."
Everton have conceded 10 goals in their opening three Premier League matches and sit bottom of the table.
They were booed off at the final whistle by their furious fans and once again face a relegation battle after a terrible start to their final season at Goodison before moving into a new stadium.
"I speak to the players all the time and that's been my biggest frustration since I walked in here. Unfortunately taking chances has been a challenge," Dyche said.
"For whatever reason they score a goal and we're just looking at each other, 'Who's going to win a challenge, who's going to win a race, who's going to win a header?' We're waiting for someone else to make a difference.
"After the first goal I could smell it in the air. Not necessarily not winning but I could smell it, 'This ain't right', and I'm screaming at them to get their shape and do the ugly side of the game. We didn't and they get a win out of nowhere."