Bugs in iOS aren’t usually very fun. In fact, they can mess up your beloved handset or tablet pretty bad if Apple’s developers have had a particularly rough patch at work. Strange as it may seem, though, happy accidents do happen as well! For all the system errors that could crash your iPhone or iPad when you needed it the most, there’s one glitch that does something completely zany and unexpected – like make all the app folders you created appear as circles, instead of rounded rectangles!
The bugger was found in iOS 9.3 and 9.3.1 and occurs when you choose to do something most users will … – Source
Bug in iOS 9.3 lets you make folder icons circular – here's how to take advantage of it