Calling getContentsMargins() for a QLineEdit does not report about the two margins used internally, look for
#define verticalMargin 1
#define horizontalMargin 2
and their uses in size calculations in qlineedit.cpp.

This frustrates one who tries to calculate with the components of a QLineEdit.

It should be also made more clear that content includes the margins around it for functions like QStyle::>sizeFromContents().

See also http://frinring.wordpress.com/2008/10/14/better-width-with-open-sources/