I've just figured out what it is. If you try to do two different bolded words in the same paragraph it ignores the middle two asterisks and bolds everything in between as well. The same is happening with italics.
e.g.
The quick brown fox jumps over the lazy dog
VS
The quick brown fox
Jumps over the lazy dog
Also: (I don't know what this does, I thought I'd experiment)
The quickbrown fox jumps over the lazy dog