When I first moved to the US over eight years ago, I thought it was very odd that people put lids on their take-away coffee. I found it quite strange to drink through a hole in a lid rather than to just drink directly from the cup, and routinely discarded the lids in order to enjoy my beverage the old-fashioned way. I thought it odder still that people would walk and drink their coffee at the same time, but it didn't take long for me to realise that this was a supremely useful activity afforded by the afore-mentioned lids.
The annoying thing was that although the lids generally did a good job of keeping coffee in its place, every so often a small amount of coffee would escape from the cup and drip onto my clothes. And with Murphy's Law in full effect, it typically happened when I had on a clean white shirt. Before a meeting.
Initially I thought it was just random manufacturing defects in the lids, or that the lids weren't on well enough. But one day I investigated further into the drippage and uncovered the culprit: the seam in the cup where the cardboard is joined together. If the lid is placed in such a way that the mouthpiece is more-or-less directly over the cup's seam, chances are that the seal will not be completely air-tight, and when you tilt the cup to take a drink the coffee will drip out of the cup and onto your freshly-laundered shirt. The solution? Simply rotating the lid to ensure the mouthpiece is directly opposite the seam appears to solve the problem – at least it has never happened to me since I started being more vigilant about lid placement.
I've never really thought this was much of a revelation – it seems like common sense, really – but having explained it to two friends recently I decided maybe it's not that obvious after all, and hence this completely non-HD-DVD-related blog post :-)
The other thing I do is to make the air inlet hole opposite the mouth hole a bit bigger using a key or pen as it means you don't need to tilt the cup quite as much.
However thanks for pointing out about the seam, I'd never thought about it.
On the white shirt front, if it's an important meeting I often have a spare stashed somewhere.
if the opposite hole is too small, sipping coffee causes a vacuum effect which makes it spurt out as your mouth leaves the lid, thus the spillage.
peter, you're so smart