It was on this petard that Clive and Gaye Derby-Lewis were hoist yesterday in Johannesburg's Rand Supreme Court, where they are standing trial with Janusz Walus, a Polish immigrant, for the murder of the ANC leader Chris Hani.
Elizabeth Motshwane, the Derby-Lewises' maid for the past five years, appeared in the afternoon as a state witness. Mrs Motshwane calmly told the court how at 8am on 6 April, four days before the assassination, she had been in the kitchen washing the breakfast dishes when the phone rang.
On the way to the phone, and again later on her way back to the kitchen, she said she saw Mr Walus and Mr Derby-Lewis sitting together in the lounge. 'I saw accused number one (Mr Walus) holding a gun downwards on the lounge table.' A court policeman handed her the alleged murder weapon - which the state claims Mr Derby-Lewis gave Mr Walus to kill Mr Hani - and she demonstrated to the court exactly how Mr Walus had held it.
Counsel for Mr Derby-Lewis, Hennie de Vos, sought to prove, after frenetic consultation with his client, that she could not have seen what she said she saw because the phone was kept in the study.
The route from the kitchen to the study, Mr de Vos - armed with plans and photos - triumphantly explained, bypassed the lounge altogether.
'Yes,' Mrs Motshwane replied, 'but the telephone is on a long cord and when they have breakfast they always move it to the passageway by the lounge.'
Mr de Vos, floored, glanced angrily back at his client, who shrugged. Driven by no more, it seemed, than the need to justify his presence in court, Mr de Vos tried another line of questioning.
'Do you know that Mr Derby-Lewis is a politician?'
'No, I don't know.'
'Do you know that Mrs Derby-Lewis is a journalist?'
'No, I don't know.'
Having established that contact between 'madam', 'baas' and their live-in employee of five years had been far from intimate, Mr de Vos judged the moment ripe to put it to Mrs Motshwane that she had fabricated her story.
Outraged, Mrs Motshwane exclaimed: 'What? Are you telling me he was not there? How could you know?' Mr de Vos, a beaten man, declared the cross-questioning over. On her way out of the courtroom Mrs Motshwane paused, turned towards the dock and gave her employers a cheerful, entirely guileless little wave.