During a discussion, one of my colleagues told that he has some difficulties with his current project while trying to solve bugs. "When I solve one bug, something else stops working elsewhere", he said.

I started to think about how this could happen, but can't figure it out.

• I have sometimes similar problems when I am too tired/sleepy to do the work correctly and to have an overall view of the part of the code I was working on. Here, the problem seems to be for a few days or weeks, and is not related to the focus of my colleague.
• I can also imagine this problem arising on a very large project, very badly managed, where teammates don't have any idea of who does what, and what effect on other's work can have a change they are doing. This is not the case here neither: it's a rather small project with only one developer.
• It can also be an issue with old, badly maintained and never documented codebase, where the only developers who can really imagine the consequences of a change had left the company years ago. Here, the project just started, and the developer doesn't use anyone's codebase.

So what can be the cause of such issue on a fresh, small-size codebase written by a single developer who stays focused on his work?

What may help?

• Unit tests (there are none)?
• Proper architecture (I'm pretty sure that the codebase has no architecture at all and was written with no preliminary thinking), requiring the whole refactoring?
• Pair programming?
• Something else?