Assuming you are one of the few people who genuinely secure their web apps using all the best known security practices like validation, authentication, authorisation etc, there is a good chance that you have ignored the common but important factor called Time.
Securing an app is fine but over time 2 things happen. Things change and things are forgotten. Why is this important in a web app particularly? Well if you used SHA-0 to hash your passwords a few years back because you knew it was the latest and greatest hashing algorithm, what are you going to do now it has been found to be less secure? Sure, you might have heard about the weaknesses and you might know that your system needs updating, but how does your organisation know that this issue will be picked up and addressed in the future?
Most organisations rely on people just knowing these things. They rely on the experience of developers or architects who have been around a while but this is simply not good enough. It is not good enough to be secure now when you consider how things can change.
In a similar way, people forget things like the fact that I used login X to access database Y for reason Z so that when someone else comes across it, they might re-use the login for something else, elevate its priviledges for some reason and weaken the overall security. How should they know to leave it alone?
A management system is required, any management system to begin with, but something which is adapted over time to be useful and useable. Make it unuseable and guess what? people won't use it. This needs to record information about the system including but not limited to security information. You might list the logins, their purpose and what access they should or shouldn't be allowed to have. It could list securables and secrets on the file system and what security they have been setup with. It should be expressly prohibited for ANYONE to modify a security setting without a review and without recording it in the management system.
It honestly could be setup on a wiki or anything as simple as that which allows regular review and work arising to be allocated. You can even flag things which we might know to be issues for the future such as encryption systems and hashing mechanisms, all of which become weaker over time.
If you don't manage security in your apps, your app is at best good for now but could break down rapidly as new exploits are discovered and no mechanisms exist to trace these to your own systems.