[radley]

The saying is something like: once you're 90% done, you're halfway done.

For code, it's because you've solved all the easy stuff and can finally see a product. It's the unexpected UX problems along with the dumb-but-hard-to-solve bugs you're now handling.

[cclogg]

Totally. I find it's especially true for UX or animation stuff. Like you get the whole application working and then animations need to be put in, but those end up causing issues with states/transitions and smoothness, which take a long time to iron out into a nice place.