The following phpbb Modifications Documentation pages are completely blank:
1.1 Introduction
1.2 History
5.1 Introduction
5.6 References
6 Glossary
6.1 Glossary
7 Appendicies

There are also sections within a chapter that are blank:
3.2.4.1.1 Assigning Block Variables
3.2.4.1.2 Using Block Variables
3.2.4.2.1 Logical Operators
3.2.4.3 Including other Templates
3.2.5 Pagination
3.3.4 Inserting Data
3.3.5 Updating Data
3.3.6 Removing Data
3.3.7 Managing the Cache

There are no Modification End-User Guides in Chapter 4.