I was so happy with the ability to FINALLY mass edit categories for products with the new 4.9.1 update.
WARNING: DO NOT USE IT!!!!
The main categoy determines the URL of the product which is, obviously, import to search engines to direct customers to your website without failing (404).
I assumed that it would only ADD categories to a product but i actually CHANGES the MAIN category, to whatever it feels like apparently. This results in a 404...
At first the SEO name (specifically the category url part) doesnt change so as long as you leave the product alone after adding categories it works fine, but as soon as you save / change the product it changes the SEO name = 404
In my enthousiasm I added an extra "secondary" category to most of our products trusting the devs they realize the importance of the main category. Mad bad. They don't. Result: lots of 404's and massive work to correct it by hand.
See attached pic demonstrating what happens before and after using the category tool.
[attachment=13582:category tool bug before.jpg]
[attachment=13583:category tool bug after.jpg]