Somehow, it never happened to me... but.

what happens if you putt on green, and you hit someone else's ball marker and the direction of your ball changes?
what are you supposed to do, if the ball marker is right on the path of where your ball must go?

I just bought a custom ball marker and its pretty thick. haha I was just worried that it would suck if someone else hit my ball marker and gets like.. double bogey cause of it. I know i'll be extremely pissed.

If you think it may be in someone's line, you ask them if it needs to be moved. They can also request that it be moved. No penalty if someone's ball hits your marker.

Better get a thin plastic white marker that bother's nobody on the green.

Simple. Ask them to remark their marker as much as you need (within reason). If you STILL hit their mark, there's no penalty for you or the marker's owner. You play your next stroke from where the ball lies.

I know one PGA club pro who uses his cell phone as a ball mark....

What a jerk!

i just use my putter face length to move the ball marker

I often play with a lad who insists on marking his ball with a tee peg...

At least you'll never hit it; because you're guaranteed to ask him to shift it if it's on your line.

I emailed the USGA about this a while back because an informal tournament I was part of had a situation where a player marked a ball with a tee and on the next players putt caused the ball to hit the tee and stop next to the hole. The USGA said that if a ball strikes a ball, you would play the ball as it lies. A ball marker doesn't have any status. It's rub of the green if a ball hits it. As has been pointed out, a player has the right to require that a ball marker be moved.