Ehehe...what?

What does that little exclamation mark over a note mean? Whenever I try to play the note it's on, it either:
a. refuses to render
b. skips the note.

What's going on and how can I fix it?

Right click. Go to 'envelope'. Then, at the bottom, click on the middle button out of the three. That should even the dots and get rid of the exclamation mark.