The special filters Canon made for ths lens have a clearance of 3mm between the rear surface of the glass and the rear edge of the rim. This is enough to allow you to screw in the filter all the way without having the glass touch the center of the front element, which protrudes farther forward than usual.
Many independent filters have less than this 3mm distance from the rim edge to the glass. At random I pulled out a 55mm Vivitar 85B filter from my drawer, measured it, and found that it has only 1.5mm clearance!
Here is a procedure for testing your filter that should be safe. Put a tiny drop of a clear liquid, such as lens cleaner, on the center of the back side of the filter. Watch the drop as you carefully screw in the filter. If you see the drop spread out, you'll know it has contacted the front glass of the lens, and you should stop!
If most of your filters are not friendly to the 50/1.2 lens, you may want to take an old, cheap 55mm filter and turn it into a "spacer." To do this, just remove the glass from the filter; usually this requires only prying out a split ring retainer. Now you can attach this glassless spacer to the lens, and screw other filters into the front threads of the spacer. The extra distance supplied by the spacer should be enough to keep the front element of your lens safe.
