We've all offered an excuse for a poor reaction or any behavior we recognize we should not have engaged in; many have rationalized it to mitigate consequences or even make it socially acceptable.
Stress, headaches, even ignorance can be proffered, but what makes an excuse plausible? In law, things are a little more clear, duress and coercion and psychological maladies have all been successful, but to families and friends it might not be as easy as convincing a jury of strangers. Especially if you do it often.
What separates a good excuse from a bad one is what a philosopher calls the Good Intention Account - though we acted wrongly, our underlying moral intentions were adequate.
Intentions are plans for action. To show that your intention was morally adequate is to show that your plan for action was morally sound. Making an excuse is a way to suggest something moral went awry in putting it into practice. Sometimes it's easy. If you drop a bag you were helping something with and you trip, you have moral grounding. If you spill the coffee after staying up all night with a crying infant, it's an easy excuse.
With other behavior it may be more difficult to have an excuse accepted by showing you had morally adequate intention but were prevented from it by fear or anger. What people perceive as morally adequate intention is as important as adequate intention.
The ingredients of a successful excuse
When we make excuses we are trying to haggle, to negotiate whether we deserve anger and resentment, or punishment and how much we need to apologize or compensate. This is why it can be so annoying if someone makes spurious excuses - and also probably why we continue to make excuses in the first place.
A successful excuse needs to make plausible that your intention really was morally adequate - but something beyond your control prevented you from translating it into action.
That's why considerations like the following often work:
I am sorry for forgetting the appointment - I had a terrible migraine
I haven't slept for the last three nights
I was preoccupied with worries about my mother's health
I'm sorry I broke your vase, I stumbled over the rug.
There's a difference between an explaining away bad behavior and an excuse
Things that will never work are appeals to weakness of will such as 'I just couldn't resist' or 'it was too tempting.' Nor do appeals to things that are obviously immoral and in legal cases, appeals to duress, coercion or provocation depend on the details of the case.
Dr. Paulina Sliwa of Cambridge notes that appealing to excuses has its limits. "Successful excuses can mitigate our blame but they don't get us off the hook completely. Saying we were tired or stressed doesn't absolve us from moral responsibility completely, though they do change others' perceptions of what we owe to make up for it and how the offended party should feel about our wrongdoing."