With a stroke of the pen, you may not only be signing off a hefty amount of money but you become an immediate hostage to fortune.
Just remember what happened to Hertz, forced to cancel its contract with OJ Simpson when the little matter of a murder rap arose. Or Pepsi, which pulled out of a deal with Michael Jackson after allegations of paedophilia.
They can pull the rug from under your best laid plans quicker than you can say Vinnie Jones. Like Sir Bobby Robson, they can get themselves very publicly fired. Not very funny if you are Barclays and have just been using the old boy to show the punters what a jolly fine Premier League sponsor you are.
It's an ill wind that blows no-one any good, though. Marketers at PepsiCo must be hugging themselves for their prescience (or downright luck) in having cast Robson and Terry Venables (who was tipped as a possible successor, until Blackburn's Graeme Souness pipped him to the post), in a Walkers crisps commercial.
Robson, you may recall, plays a benevolent angel urging Gary Lineker not to steal a packet of crisps while Venables, dressed as the devil, urges him on. All rather apposite when you consider the £3.5 million-worth of pay-offs Tel has trousered since 1998.
Then there's poor Paula Radcliffe. Didn't our hearts go out to her when she collapsed weeping during the Olympic marathon? Didn't we feel for her when she broke down again in the 10,000 metres final?
Not half as much as the people down at Quaker Oats, who, according to the weekend papers, were "chuffed to bits" at getting her agreement to star in their new TV commercials.
Quaker said a decision not to proceed with the ads was because the BACC would not pass the scripts. Glad to hear it had nothing to do with the strapline: "Quaker Oats - helps you go the distance."