It'll, throw a tantrum, drop all its feet on the floor, and swear at you!
Seriously, nowt! Nada! Nothing at all! I frequently use different threads top and bottom, depending on the effect I want. For general seaming, to get a nice even stitch locked where the two layers of fabric meet, using the same thread top and bottom is best. HOWEVER, there are plenty of circumstances when that is not possible, not advisable, undesirable, or you just plain don't want to! You may have to tweak the tension to get the result you want, but that's all part of the fun! :)
Some places where I use different thread on top and bottom include:
Buttonholes: if I'm doing them with embroidery floss, I use either a bobbin thread designed for embroidery in the bobbin, or 120's poly - serger thread!
Embroidery: see above! Bobbin thread is MUCH cheaper than embroidery floss, and you won't see it anyway...
Quilting: I use a thread that will either tone or contrast with the top, depending on the look I want, and quite often a completely different one in the bobbin because the back is a different colour!
Different coloured fabrics: if I'm sewing something that is red on one side and blue on the other, I might match the thread to the side it will show on: useful for reversible garments! :)
Topstitching: topstitching thread is very heavy and you want it to show, but it doesn't need to show on the back, so use a polycore or standard thread to match the garment. Just look at the way jeans are topstitched...
I have 9 working sewing machines, and have tried this trick with all of them, and it has been successful every time. Just give yourself time to experiment and get the look you like. For buttonholes and embroidery, you want the stitches to lock on the back: for seaming it needs to lock in the middle. That's just a matter of tweaking the upper tension.
If you are using heavy thread in both bobbin and needle because you are sewing heavy fabric, you may also need to adjust your bobbin tension to cope.