The Magpies goalkeeper was beaten four times inside the opening 26 minutes as Arsenal claimed a seemingly unassailable lead at a shellshocked St James' Park.
However, like the rest of a capacity crowd, Harper watched on in amazement as Newcastle proceeded to score four goals of their own in the final 22 minutes to claim a remarkable point.
The 35-year-old has witnessed plenty of unexpected occurrences during his 13 years as a member of Newcastle's first-team squad, but nothing could have prepared him for Saturday's incredible second-half spell.
It's probably the greatest comeback in the club's history, said Harper.
It's certainly the best game of football I've ever been involved with.
I remember the Leicester game (in February 1997) when we were 3-1 down, but Alan (Shearer) scored a hat-trick and we won 4-3. This was even more incredible than that though.
I've been three-nil down at home before it happened against Portsmouth a few years ago (when Newcastle lost 4-1) but I've never experienced anything like this. To come back and draw was unbelievable, but we kept on going to the end.
Overhauling a four-goal deficit would have remarkable enough whatever the circumstances, but the context of the build-up to Saturday's game makes Newcastle's achievements all the more notable.
The sale of Andy Carroll on transfer deadline day, and the subsequent failure to sign a replacement, knocked the stuffing out of everyone associated with the Magpies.
The prevailing mood on Tyneside worsened when a lifeless Newcastle slumped to a 1-0 defeat at Fulham on Wednesday, a defeat that also saw striker Shola Ameobi suffer a fractured cheekbone that will keep him on the sidelines for up to two months.
Had Alan Pardew's side capitulated further in the second half of Saturday's game, perhaps going on to record a five or six-goal defeat, it is easy to imagine the extent of the acrimony that would have been apparent at the final whistle.
With only five points separating them from the bottom three, the Magpies could still find themselves embroiled in a relegation fight. At least, after Saturday, they know they have the spirit and commitment to survive it, something that was not the case when they careered into the Championship two seasons ago.
People talk about buying this and selling that, there has been a lot of focus on that here for obvious reasons, but you can't buy character and that is what we have in abundance here, said Harper.
It would have been so easy to crumble, given everything that has gone on, so you can't underestimate what a big point that is for us. The days and weeks of doom and gloom (have been avoided).
Monday wasn't great losing Andy, and we weren't great losing at Fulham on Wednesday. People were writing us off before the game and they had probably written us off at half-time, but when you get the crowd behind you going down that bank, there is a chink of light.
We got on the front foot and kept our shape. We were organised and didn't concede again, a lot of comebacks are killed off when the other team scores again, but we were disciplined at the back.
That discipline had not been apparent in the first half, of course, but while a handful of supporters left at the interval, the majority of the St James' Park crowd remained to support their team.
They were rewarded with a half-hour spell that will live long in their memories, and having experienced the Gallowgate roar on countless occasions in the past, Harper is only too aware of the importance of Newcastle's raucous support.
The crowd never turned,
he said. It could have been anti-this and anti-that, but they were fantastic. This place can be a roller-coaster, as I've said before, but on afternoons like Saturday, there isn't a better place to play football.
There were some angry fans at half-time and rightly so, but we have fantastic support who get behind the team as long as they see you giving your all.
We got that on Saturday, it was a special occasion. You could see Arsenal's players wilt a bit. We stood off them and they hurt us, but with the fans backing heading towards the Gallowgate, somehow we got back into it.
Brazilian free agent Ewerthon has expressed a desire to join Newcastle, having been released from his previous club, Palmeiras, last month.
Pardew is hoping to sign an unattached striker to bolster a squad that could be without Leon Best on Saturday after the Irishman injured his hamstring at the weekend.
Ewerthon is one of the players currently under consideration, and the South American's agent, Barry McIntosh, claims his client, who scored seven goals in 21 games for Palmeiras prior to his release, is keen to move to Tyneside.
A number of clubs like Ewerton, he is a free agent and is looking to seal his next move,
said McIntosh. It is no secret that a number of Premier League clubs are keen on him.
I am not going to go into details about who we are talking with, but if you're asking me about whether he would move to Newcastle then yes, obviously they are a calibre of club he would very much consider playing for.