I hate when you reach the point just beyond your personal caffeination comfort level. For me, it's about cup number four, depending on the brand and the roast.
Coffee ceases to serve its purpose when you're typing 150 words per minute but have to go back and delete a typo every third word. When your drink sloshes over the rim of the cup every time you try to set it back in the saucer delicately, you're in trouble. And when you can't make your eyes focus on anything but the line where the walls meets the ceiling, a line that appears to be jumping up and down, you've reached a point of no return. Instead you have to sit and be ineffective and wait for the inevitable slump, which feels like sobering up from a mid-afternoon bender.