CS6: Single black frame between two clips shouldn't be there...

Hi,

I hope somebody will be able to answer this question.

I have two clips in my timeline that I have snapped together, on the timeline there doesn't appear to be a gap and yet when I advance frame-by-frame a single black frame appears.

I have tried cutting a frame off each of the clips but the black frame still appears.

Here are some screenshots to further clarify (I am zoomed in all the way on the timeline).

so this is the final frame before the split

this is where the two clips join

this is the first frame after the split

I'd appreciate any help/advice that anyone can offer!

P.s. I'm running Premiere CS6 on Mac OSX 10.8.3 (Mountain Lion)

Thanks!