Well I did not found any bugs except that floating mouse.

About the graphics, I understand that you don't want to recreate e.g. Doom-like graphics, but as it is, it is not really exciting to have a weapon that is made out of 4 different pixels, most importantly cause you can't make out what it really is. Now if you would give the machine gun weapon a similar resolution as e.g. this , one could atleast see clearly it is a machine gun (I am only talking about the weapon on the picture not other things).

Concerning gameplay, consider adding (semi-)big areas too (with more enemies) and more powerups.