If your drive gets too full of bits - the bits will start spilling on the floor. I don't know about you - but I hate cleaning up spilled bits. (ok - just kidding).
Your system will likely complain before you get there - however if it does not - then you may have a hung process when the last bit is written and more remain.
There may be a good reason your drive is filling fast - junk files and recycle bin are primary culprits.
Also if it is old, then you may have a number of bad sectors - which when identified, are taken out of the available storage.