In MicrosoftSecrets (ISBN 0-02-874048-3 ,Chapter 5, pages 280-281), Cusumano and Selby  point out that development managers at MicrosoftCorporation try
to allocate about 20 percent of developers' time to rework weak parts of the product. As an example they state font availability in MicrosoftWord, where the
maximum number of fonts was changed from 256 to 4,000. The main aim is to improve the fundamental structures of the product, rather than to contribute directly to
the development work. They state that this work tends to get done immediately after shipping a complete release. --

See also TechnicalDebt