Lets say it is 1:00 pm on june 1st.
Your in your basement polishing your time macine. You finish at 1:01 pm.
Now you hope in the machine and goto june 1st at 1:00 pm, the time you where polishing your machine.

Now your back in time looking at yourself polishing the machine. As you keep watchine your past self, you see him go in the machine back to june 1st at 1:00 pm.

Would a infinate amount of you pop up? What do yo think would happen.
This is a reason why I think time travel is bad.