The champions apparent arrived at Vicarage Road on Saturday having gone 44 league matches unbeaten, just five games short of the record set by Arsenal’s “Invincibles” as they went through the 2003-2004 season without loss.

But Jürgen Klopp’s men, whose last Premier League defeat had occurred 422 days earlier, were obliged to drop their record ambitions as they lost 3-0 at the very same ground where the club suffered what remains, arguably, its most surprising defeat.

That calamity for the Kop happened on February 21 1970 in the quarter-finals of the FA Cup.  And I, along with the majority of my class-mates, was there to watch it.

We had our young first-year form master, Mr Peter Thompson, to thank for this bumper outing. With what now seems like extraordinary faith, he decided that the arrival in our parish of the still mighty Reds, guided by the passionate managerial genius of Bill Shankly, should be marked by a school trip that involved at least a dozen voluble, volatile 12-year-olds.

Imagine the obstacles that would stand in the way of such an optimistic enterprise now.  Actually don’t even start because there are too many.

How did Sir, with his horn-rimmed glasses and pepper-and-salt jacket, get the tickets? Presumably our parents paid for them. I can’t remember because, frankly, I didn’t care.

All I cared about on that distant Saturday afternoon was getting a decent view over the fencing at the front of the old Shrodells Stand terracing for what turned out to be almost an unmanageably exciting occasion.

The site dedicated to Liverpool’s playing record, lfchistory.net, carries a picture of the front of the Watford Observer’s FA Cup special edition for the match, with Oliver Phillips, who covered everything that moved at Vicarage Road in those days, writing the main piece under the headline: "Be Prepared For Surprises - It’s all too quiet…"

By the bye, local outfitters J. P. Taylors also vouchsafed that the Watford manager, Ken Furphy, wore Rocola Golden Tricopress shirts in Bri-nylon.

To be fair, a year earlier Watford had earned a draw at Old Trafford before losing to Manchester United in the FA Cup fourth round.

And they had already beaten a First Division side on their latest Cup run, having earned a 1-0 win over an ageing Stoke City side in the fourth round.

But taking on Liverpool, whose league victories in 1963-1964 and 1965-1966 and FA Cup win in 1965 had launched them into a series of heady European football campaigns, was a very different proposition.

The Observer special also carried the following quote from Liverpool forward Bobby Graham after the quarter-final draw had set up a trip to the newly-promoted Second Division side: “I can’t honestly think of anyone else we would have preferred to play. Obviously it would have been better if we had been drawn at home.”

I can only say that the gathered members of 1THO did not share Bobby Graham’s confidence about the outcome. And that judgement, based on blind optimism and ignorance, plus for some a vague memory of Stewart Scullion’s wonder-goal at Old Trafford, proved correct.

Around the time of those Manchester United matches, some older boys I knew would sing a song comparing Scullion and Watford’s keeper of the time, Bert Slater, to world stars Eusebio and Lev Yashin.

“Aye, aye aye aye, Slater is better than Yashin, Scullion is better than Eu-se-bi-o, United you’re in for a thrashin..."

For most of us, attending proper football matches was rare. I was spending as many of my waking hours as possible playing the game, but I had only seen my local team a couple of times in the previous year or so.

Watching Watford lose 1-0 to Stockport County in the Third Division - to a goal by Jim Fryatt just in case you were wondering - was one of the highlights of my life at that point. So can you imagine the anticipation levels for watching a team featuring legends of the game such as Ron Yeats, Ian St John, Emlyn Hughes and Ian Callaghan?

Beyond all that, the idea that you were allowed to stand with your classmates and shout as loudly and for as long as you wished without being told off was giddily exciting.

And how we shouted as, 63 minutes into a game where Liverpool had been relentlessly penned and harried by eager, golden-shirted Hornets, Barry Endean broke through audaciously to drive the ball past Liverpool keeper Tommy Lawrence - known unkindly to some as "the Flying Pig" - for what proved the only goal of the match.

Watford had beaten Liverpool. Watford had beaten Liverpool.

The lfchistory site also carries a cutting from a local paper report written by Norman Wynne, whose opening paragraphs read:

“The occasion demanded something special from the no-hopers of Watford. And, by golly, they supplied it with a display of aggression which shattered Liverpool’s Wembley dream.

“Liverpool, the Cup aristocrats, were made to look distinctly the poor relations throughout a game which gallant Watford, eager to reach their first-ever semi-final, dominated with brave hearts and determined running…”

Onwards went Watford, to a semi-final at White Hart Lane against Chelsea. At halftime, with the score at 1-1, Wembley still shimmered on the horizon for the Hornets. At the final whistle, with the score at 1-5, it didn’t.

As things turned out, that seismic shock for Liverpool provoked one final reformation of the territory under Shankly’s charge as he brought new young players into his side for what would prove to be a glorious finale to his managing career, culminating in League and UEFA Cup wins in 1973 followed by one of the most overwhelming and free-flowing FA Cup victories ever seen.

Liverpool’s latest upset at Vicarage Road will most certainly not provoke a similar re-stocking initiative from Klopp.

As for me - I've still got the match programme.