Before formulating a set of best practice rules on drafting conditions, I will discuss one final aspect in that context: the waiver of conditions by a party.
Whilst a condition provides for an exit right for one party in case of non-fulfilment, the opposite is often true as well. For example, if the closing of a transaction is conditional upon certain conditions being satisfied and both parties fulfil all the closing actions, the closing itself implies a waiver of any pending conditions. This would be the effect of the Lex Mercatoria principle that “no-one may set himself in contradiction to his own previous conduct (non concedit venire contra factum proprium)”. Clearly, if a party pursued the closing on the pretext of the other party’s statement that all conditions were satisfied and it appears that a material adverse change has occurred (triggering a MAC-condition not to be satisfied), the legal concept of fraud or even mistake would imply a right to reverse the closing and to claim damages. This is generally addressed in the lead-in to the conditions, where it stipulates that:
each of the following conditions be satisfied or waived.
Although generally there is no need to express this ‘waiver effect’, it may well trigger a contract party to require a positive statement (in the US context often called a “certificate”) confirming that a fact or an event (as foreseen in the condition) is absent or has not taken place, respectively.
 This generally recognised principle is included under No. I.7 in the CENTRAL list of Lex Mercatoria principles, rules and standards (click here for the full list of the principles). The list is included in Willem Wiggers, International commercial law – source materials, Kluwer 2006.