I've noticed these bugs in Visual Studio, but they are probably evident in many other apps too.
1) The mouse icon does not correctly change color to show itself on any background. Normally with text on a dark background the mouse text cursor changes to white. On Visual Studio if you set a dark background (e.g. green code on a black background) the text cursor will change to white whilst over the text. With Fusion, the cursor stays black regardless of the background color, making it impossible to see the cursor at all whenever there is any text on a black background.
2) In the GUI editor in Visual Studio there is a Toolbox tab on the left of the screen that pops out when you mouse over it. It has all the widgets for designing apps and normally you pull the widget you want off the toolbox and drop it onto your Form/Panel/Whatever. When doing this in Fusion there is an easily reproducible mouse bug that occurs.
If you drag any widget off the toolbox directly onto the form then the toolbox tab correctly slides away to hide itself and you can drop the widget onto your form fine. However, if you drag any widget off the Toolbox and do not immediately enter the Form area then the mouse icon starts flickering constantly between two different states and the Toolbox does not disappear. If your form is completely hidden by the toolbox then it becomes impossible to drop widgets onto as the toolbox never slides back in.
See the first attached pic for further explanation.
3) When toolboxes do slide back correctly they often leave redraw errors on whatever was underneath. This can be seen in the second attached image in which it is clear that the Exit and Tools buttons, that are normally a very smooth texture, are quite messed up.