Well, it had to end sometime - and it has today with an emphatic 4-0 Hearts win.
The Scottish champions have been dominant under Brendan Rodgers, but no-one could have possibly predicted what would happen on a cold Sunday in December on an away day to Tynecastle to face fifth-placed Heart of Midlothian.
The last time Celtic found themselves 3-0 down in domestic football was five years ago against Rangers, coming back to 3-2.
But that's just what happened on the 47th minute, with Hearts putting the game to bed shortly after halftime.
The last time they were 4-0 down to a Scottish team? Against St Mirren in Tony Mowbray's last game in charge in 2010. That happened on the 75th.
Today, the Invincibles quickly became the Invisibles as Hearts ran rampant. The game was a battle throughout the first half, making it even more surprising that the opening goal came from 16-year-old Harry Cochrane when he lashed home a powerful low drive from outside the box, beating Craig Gordon at his near post.
The teenager showed incredible maturity with the strike, setting up a very unpredictable day. The goal came as a direct result of a misplaced pass by Celtic centre-back Boyata.
The next goal was maybe less predictable, with former Rangers man Kyle Lafferty cracking in another low drive from outside the box, this time hitting the far post and trickling in - again, a result of poor defending.
You would be forgiven for thinking that the game still had plenty to offer with time to go, and perhaps expected a quick goal after halftime to set up a tend second half.
Well, we did get a goal immediately after halftime, but it was Manuel David Milinkovic who dribbled through the Celtic defence and put the game beyond Celtic.
Celtic did have a few chances to come back with some decent attacking play, but the so-called Unbeatables failed to trouble the Hearts goalie.
Milinkovic then doubled his tally with a penalty, after coolly slotting it away from the spot. Gordon went the right way but a low, powerful strike completed an incredible team performance by Hearts and compounded Celtic's misery. Milinkovic, on loan from Genoa, could have even had a hat trick on a couple of occasions if not for some fine goalkeeping.
The game was pretty even if you look at everything but the scoreboard, but Hearts took their chances and Celtic were miles off the pace in the final third. Their defence had looked shaky in previous games, but I don't think anyone expected this.
Celtic have had their fair share of draws this season, with five so far already, but this rampant Hearts performance delivers their first domestic defeat in 69 games, and possibly just gives Aberdeen and Rangers a little glimmer of hope that the league may not be as predictable as it once seemed.