A brace from Danny Graham, either side of Jordon Mutch's fortuitous strike, secured a surprise win for Watford as Rangers blew the chance to go seven points clear at the top.
There was a silver lining for manager Neil Warnock, though, after he promised early in the season to continue to wear shorts on the touchline until they lost their first league game.
The 62-year-old probably did not envisage he would still be bare-legged in mid-December, some 19 matches later.
But he can finally pack some warm trousers after watching Watford tear his side apart in front of the TV cameras at Loftus Road.
The hosts actually twice came close to opening the scoring in the first five minutes when Jamie Mackie shot inches wide and former Hornet Heidar Helguson's volley brought a fine save from Scott Loach.
Warnock's temperature rose considerably moments later when John Eustace appeared to handle a cross in the area but referee Darren Deadman was not interested.
Rangers survived a scare in the 11th minute when Will Buckley scampered through on goal but Paddy Kenny spread himself well to keep out the shot.
But the Hornets took a 26th-minute lead when Andrew Taylor overlapped down the left and his deflected cross fell at the feet of Graham to fire home.
Four minutes later the visitors were two up when Mutch's speculative curler deceived everyone, including Kenny, and ended up in the back of the net.
Rangers captain Adel Taarabt was having one of his more eccentric evenings, but he came close to pulling one back when his curling free-kick flew narrowly wide.
However, Rangers were indebted to a brave block from Clint Hill, who flung himself in front of another Mutch shot from eight yards out to keep the deficit to two just before the break.
But the Hornets did grab a third three minutes into the second half when, with Rangers appealing for an offside, Graham burst clear again and tucked his shot past Kenny.
It could have got worse for the hosts, with Shaun Derry heading another Mutch effort off the line with Kenny beaten and Graham somehow shooting over from underneath the crossbar.
Ex-Watford striker Tommy Smith sidefooted a consolation for the hosts with two minutes left but the Football League's only remaining unbeaten record had long since been shattered.