Setting the line height too tightly can result in overlapping lines of text in this situation. (w3.org)
The artisan prints overlapping lines of tangerine ruby red maize yellow and royal blue on a warm white background. (polyvore.com)
Those smudges, which I normally don't even notice unless the iPad is asleep, become more apparent (and annoying) when they consistently overlap the lines of text you're reading in a digital book. (macworld.com)