1. Single line using the double slash:
// This is a comment.
2. Multiline (also called block) using the slash-asterisk combination:
/* This comment can go over multiple lines. */
/* some code var x = 'some string'.search(/[a-z]*/g); some code */
That’s obviously a useless example in itself, but it shows a situation in which a problem is caused by using block comments.
All that being said, I wouldn’t suggest that you avoid using multiline comments. They’re quite handy, both for adding lots of documentation and for debugging purposes (by making blocks of code inert). But the potential conflict is something to be aware of, particularly when you apply multiline comments to render some code non-executing and all of a sudden have a syntax problem.