Fortunately, I noticed before anyone else did so I made a quick escape to the toilets to fix it up.
I was wearing one of those shirts where it's not overly noticeable if it's inside-out (it has embroidery on the front of it and therefore also on the reverse side), but I was sitting at my desk looking at it and thinking the embroidery looked a bit different to normal. Then I thought the tag at the back of my neck and the little ribbon hanging things on the shoulders (the most annoying bit of any shirt) felt like they were in the wrong place.
The clincher was when I put my hand down by my side and discovered the little white washing instructions label...on the outside of the shirt.
Oops. It could have gotten embarrassing had anyone else noticed, like a client for instance. And I wasn't quite brave enough to share my little mistake with the other girls in the office, even though we probably all would have had a good giggle about it.
But, oddly, it seems fine to put it out there on the www on my blog for anyone to read...