Scintilla is a free source code editing component for Win32 and GTK+. As well as features found in standard text editing components, Scintilla includes features especially useful when editing and debugging source code. These include support for syntax styling, error indicators, code completion and call tips. Styling choices are more open than with many editors, allowing the use of proportional fonts, bold and italics, multiple foreground and background colours and multiple fonts.
DLL Hell is a good bad example
For a good example of problems with modularity look at Windows DLL Hell. Many applications depend on shared System (or semi-System) DLLs. They run fine with the versions of these DLLs that they are tested with but then installing another application replaces the shared DLLs with newer (and sometimes older!) versions. The new version has a new bug or requires stricter compliance with an interface contract and so blows up.