Honestly? Never... I tend to think of my SOTD way too deeply before deciding on it.
Every now and then. My mood changes, my plans change or something just ends up feeling a little oppressive, or slightly insipid. It happens.
Quite often. A lot of fragrances that I like make my nose hurt and I'm miserable. Sometimes with a new fragrance I don't realize this for a couple of hours and then it is too late.
Hardly ever. My "secret" is not to plan/decide beforehand what to wear but to pick what I'm in the mood for.
"Wovon man nicht lesen kann, darüber muss man schreiben."
Happens to me two or three times a year. The regret is usually related to weather or activity - I end up in a hot room, or find myself working up a sweat while wearing something that doesn't work well with heat - or I'm sampling something that I end up not liking. Today, for example. It's D&G The One Gentleman. Somebody somewhere likes it, I'm sure. It's definitlely not for me.