This was a game they should have won comfortably, but instead they let slip another lead thanks to horrendous defending, with Panthers centre Travis Robinson scoring at late dagger of try when he looked to be covered.
The other bad news is they lost hooker Nathan Friend to what looked like a serious arm injury in the first half.
Friend has been one of the Warriors' most consistent performers in recent weeks and in his absence they had only Shaun Johnson to provide anything in the way of inspiration.
This defeat was a gut-wrencher for McClennan and his team, who played like they were unaware of the scrutiny being placed on their coach in his first season in charge.
The fact that the recently sacked Parramatta coach Stephen Kearney was watching at Mt Smart as a guest of the owners would have only increased the stress levels of the man known as Bluey.
In a twist, it was former Warriors coach Ivan Cleary back in Auckland to turn up the heat. But the performance his side - dressed in pink - wasn't pretty either, but they get their act together eventually.
The Panthers, who began the match 15th on ladder (two below the Warriors), bombed at least three tries in the first half due to poor handling or wrong options, but finally contrived to get it right three minutes before the break when captain Kevin Kingston strolled under the posts to narrow the score to 10-6.
Ben Henry had opened the scoring for the Warriors when capitalising on a James Maloney kick and a mistake from fullback Michael Gordon, and halfback Johnson ran in from 80m following an interception.
Henry's second try six minutes after the break lifted the enthusiasm levels, or maybe the ears of the men in black were still stinging from a halftime talk. Whatever, the Warriors were finally playing with purpose.
Alas, it wasn't to last. The defensive frailties of the Warriors, who let slip two 18-point leads to lose in consecutive weeks and followed that with thrashings at the hands of the Sharks and Cowboys, soon reappeared when Lachlan Coote shrugged off several would-be tacklers to score the softest of tries.
Then came Robinson's try down the left sideline, converted by Luke Walsh. The Warriors tried to reply in a frantic last couple of minutes, but it came to nought, much like their season.
