What is it that makes things go wrong more often on Thursdays?

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Yesterday I was donating blood plasma and the machine malfunctioned. Instead of separating out my red blood cells so they could be returned, it spilled them throughout the (single-use) blood-handling parts of the device. So I ended up losing two pints of whole blood, instead. And because the contamination made it into the plasma bottle before anyone noticed, it's probably unusable.

So today I'm out of sorts due to blood loss, and grumpy 'cause it won't even do anyone any good.