After reading some horror stories from YLF, I now wash everything before I wear it. Which makes it tricky with the stuff that I haven't worn yet - do I cut off the tags before I'm sure, or do I wait but then I have a delay before I can actually wear it? Like Suz, I've had mistakes in both directions - cutting off tags and then regretting it, and returning stuff that I later want back.
Usually I try to let things I'm unsure about hang out in the closet for a few days, so that I can take some time on the weekend or on a less busy weekday to try it on and demo some outfits. Also, if it was expensive or unplanned, I like to let my monkey brain fade a bit and take a sober look at my budget before cutting off tags. Then there's the third limiter: when am I going to be in the area where the store is? I much prefer to run errands in serial instead of wasting a trip on just one return.