Any plumbers in the house?

[mintacular]

So, I've done some research via google but thought I'd try here...I had a running toilet and determined that I needed a new fill valve as the water would not stop rising in the tank and was being rediverted in the bowl....So, since I've been working a lot this week I just turned off the shut off valve (the silver oval shaped handle) and figured that when I have a day off I would install a new fill valve. Well, I wake up this morning and there is a small hole in my ceiling with water leaking down from the toilet area and a modest water blob around this hole! So I freak out run to the hardware store and buy a new fill valve....Install it and turn back on the shut-off valve. When I did this the leak stopped in the living room and also the toilet is now functioning fine with no extra running water. My question is why did a leak a occur when I had the shut-off valve "off" and do I need to fix this or as long as it is on the "on position" am I fine? Any ideas? Thanks!