A business analyst colleague asked me to comment on how he could improve a use case diagram he had inherited. Someone had carefully transposed the outcome of a whiteboard session into a Visio UML diagram. We discussed the difference between <<includes>> and <<extends>> and I whinged on about how 'traditional' UML use case diagrams don't always convey very useful information. I made one practical suggestion, though. The use cases and associations were all neatly colour-coded using two colours (red and green). However, I couldn't tell what these colours signified, so I suggested that he annotate the Visio diagram to make the semantics clear.
It turns out that the colours reflect the fact they had two different whiteboard pens.