I've come across this a lot in my working life, experiencing it myself and witnessing it. It's a fairly generic question and many different circumstances need to be considered (nature of the job, employee relationships, reason for stress, reason for lashing out etc) but in general, is there ever an excuse for it?
When I say 'losing your temper' I mean being offensive or overly aggressive to a colleague. I'll give you an example.
A customer calls you up and chases you for an update on the server status. Network connectivity is down and engineers are on the job. You politely explain that you're working on it and it should hopefully be back up and running soon. You then get a barrage of abuse down the phone like 'you're worthless, what do you people do all day? Let me speak to your director'. They then e-mail you later and apologise. They say they've been under a lot of stress from deadlines.
Yes, credit to them for apologising, but should it have happened in the first place?