Legacy system development

If you're like me, then you probably faced developing new features for legacy systems (system with no test coverage) without full understanding of source. I've done this a couple times, and want to share lessons learnt:

Legacy documentation

There are 3 sources of information about system - experienced colleagues, documentation(if exists, including javadocs and tests) and source. Fact is, you should not trust first two and use them only to understand third.
Colleagues may forget smth or think of it as of obvious thing. Tests, if they exists may be incorrect or deprecated. If documentation isn't a part of delivery, noone bothers keeping it up-to-date.

Development surroundings

Most likely new development will introduce bugs. Mine does. But not in feature you're working on. It will be some other functionality, linked with yours.
So, you need to recheck code you're depending on and recheck code, that depends on your new logic.

Also requirements is important - if you haven't understand them, you will not deliver valuable feature.

Summary

• Use all information you can get, but dont rely on it
• Only reliable source of information - code
• Check and recheck code you're depending on and code that depends on your's code
• Understand your requirements