A particularly humiliating loss to a rival, local or otherwise, can be too much for the club's board to bear, leading to a knee-jerk reaction and a tear-stained letter-opener through the heart from chairman to once-beloved coach.
Most perilous of all such fixtures has been the 2013/14 East Midlands edition. At their first Sky Bet Championship meeting in September, it was Billy Davies and Nigel Clough eyeing each other up along the touchline. Then, it was Davies who emerged with victory for Nottingham Forest, Clough who was sacked by Derby County.
On Saturday, the reverse fixture took the scalp of Davies, after his side capitulated to a tame 5-0 defeat - with Scotland internationalist Craig Bryson scoring a hat trick - at Pride Park.
How Davies must hate the sight of Steve McLaren. When the Scot was sacked in his first spell, McLaren sidled into the City Ground the next day, his knowing, tanned grin and glamorous international savvy too seductive for the dazzled Forest board to pass up. His part this time was more direct: Davies' team looked lost on Saturday, broken and out-thought by McClaren's reborn Derby.
Even so, Forest are sitting just two points outside the play-off places. Whoever Davies' successor may be - Neil Warnock has ruled himself out - will surely not believe his luck, walking into a plum job, with a squad of talented players.
Davies' record is good. In five full attempts - discounting a couple of half-seasons in charge at Forest - he has never failed to take a side into the Championship play-offs.
His major failure in that time, was ironically his biggest success. Against the odds, he took a poor Derby side to the Premier League in 2007 and, as expected, they imploded spectacularly without the help of investment.
For all that his second-tier record stands up to all scrunity, though, it is what comes with it which sours his reputation.
Despite the league consistency, Davies remains unpredictable. When he made his jubilant return - after 20 months out of the game - to giddy acclaim at Forest, he was backed by the board, allowed to invest and given full control.
Yet still he never seemed happy, perhaps too keen on settling old scores to allow himself - and the club - a true fresh start.
He has a quickness to anger that seems almost eager at times. Camped behind sandbags in his now-departed office, photographs of former grudges pinned on the wall and criss-crossed by manic string, you imagine Wild Bill with his fingers twitching close to his holsters, poised to pull the trigger.
After Saturday's match he failed to clap or even acknowledge the away supporters who, in that black-humoured way, sang throughout as their team folded. It was Davies' assistant Ned Kelly who was seen to be ushering the players, making sure that they, at least, showed the fans some gratitude.
A sheepish Kelly conducted the post-match post-mortem. A pending touchline ban appeal, and some accompanying legal advice, had apparently stopped Davies from facing up to defeat, just when a strong public voice was required.
Perhaps he already knew then he was a dead man walking.
But Davies has never enjoyed a healthy relationship with the press. In truth, he has never looked like a man who understands how to work the media, or at least how it is sometimes best to bite the tongue and deflect attention rather than be tempted blindly to the flame. "Job done," he famously said after the first leg of the 2007 play-off semi- final at Leeds after walking out of Elland Road victorious.
It wasn't, of course, and those two words pinned up on the wall helped to fire the Leeds comeback.
More recently, media blackouts and bans for individual journalists have followed criticism this season. Conducting a post-match press conference - as he once did against Leicester - before kick-off was only ever going to fuel resentment and bitterness from those who - rightly or wrongly - help to shape his public image. In the wake of yesterday's sacking, the Forest chairman, Fawaz Al Hasawi, immediately ended all media restrictions imposed by Davies.
For all the hoopla, though, there is no denying that he really has something. In one of the toughest leagues in world football, he has barged and bullied his way to consistently high finishes without even a single outlying failure.
At the same time, though, Davies has proven himself to be neither canny nor subtle, and that is what may prove to be his downfall.
Derby defeats are dangerous. It seems a Derby defeat has proven fatal. So, where next? Clough, who left Pride Park with his reputation more or less intact, has an FA Cup semi-final to look forward to with his new club, Sheffield United.
But it took Davies 20 months to get back into football last time, and he ended up back where he started.
There is a queue of managers scavenging for work at that level - just look at how Owen Coyle's star has fizzled out - though you might expect that in a time of crisis some chairman might take a chance on a wildcard with a good record.
If not, he may have to really bide his time. Davies has been linked with Rangers in the past, making no secret of his desire to manage the club. Should the wheels come off the Ibrox side's relentless push up the leagues, this 'Championship specialist' might keep half an eye on a version closer to home.