There has been a long-running dispute in the civil courts as to whether compensation for a breach of contract or a tort should be calculated from facts known at the date the wrong was committed (based on the idea that it should be foreseeable) or if information discovered subsequently can be taken into account. In what has become known as the Bwllfa principle, the House of Lords has confirmed in a series of decisions that the latter is to be preferred.

However, the application of the Bwllfa principle to employment tribunals has resulted in controversy. In particular, in McDougall v Richmond Adult College, the Court of Appeal overturned the decision of the Employment Appeal Tribunal that it can be used to determine the question of whether an impairment is likely to recur, which could allow it to qualify as a disability. It stated that the Bwllfa principle applies only to the assessment of damages rather than to liability and jurisdiction issues. However, it remained silent about its application to the calculation of compensation for unfair dismissal, which is just and equitable and so departs slightly from contractual principles.

In Aryeetey v Tuntum Housing Association, any lingering doubts were resolved by a decision that an act justifying the dismissal of the claimant would bring his losses to an end, even though it took place between the liability and the remedies hearing, and even though the tribunal had hinted, at the end of the liability hearing, that it saw no reason for making a Polkey reduction. The Bwllfa principle was not explicitly mentioned but McDougall was cited.