For whatever reason, it's something that I've had to battle with in the web design world quite a bit: Just one more tweak.
I'm not the world's greatest web designer, nor am I really a great designer. I'm adequate. I understand current standards and now how to update sites to meet them. I follow any upcoming or recently updated standards and enjoy implementing them moving forward.
However, once the design is more or less complete I just can't roll over the progress from 99% to 100%.
"Oh! There's too much whitespace on IE7 and outright eliminating it affects all the other browsers, so I'll spend an extra handful of hours making it look exactly the same on every browser." I'm sure that IE7 users would appreciate the effort, but I'm also pretty sure that most people using IE7 experience the web through broken glasses anyway.
"Oh! That font isn't sharp enough, let's go to Google's webfont directory and spend a couple hours picking the perfect sans-serif." Just no. I don't know fonts and just learned what serif and sans-serif meant like a year ago.
Things have gotten better, a combination of me consciously trying to achieve this and dealing with the business world. Timelines exist, you gotta know when to say 'enough' and if you don't, well, people will say it for you. So in order to have complete control over my projects, it was prudent to define what 100% complete was. Sure, it wasn't literally 100% complete, but it was in my eyes.
...That said, thank god for Twitter Bootstrap.