Twitter tells us in their release notes:

Better box model by default. Everything in Bootstrap gets box-sizing: border-box, making for easier sizing options and an enhanced grid system.

This sounds so cool. But in real life this could lead to problems with old-fashioned CSS apps. They did not apply the new box-model to single elements, they changed it hard and global for everything:

*,
*:before,
*:after { box-sizing: border-box; }
/* <a href="https://developer.mozilla.org/en-US/docs/Web/CSS/box-sizing">box-sizing</a> */

When you use a hard-coded height or width in combination with a padding or border for elements, you need to check if it still looks the same.

Previously styled form elements are now a bit smaller if you don't fix them. When starting with new Bootstrap 3 stylesheets, there are no problems to be expected.