I'm sure I read or heard this somewhere before but my searches have turned up nothing.
If you fix a bug or add a feature to an open source project and you don't contribute that bug back to the project this is referred to as the stupid tax.
The reason that you fixed this bug in the first place is because it was preventing you from using the open source software the way that you wanted to use it. It might not even be a bug, it might be a feature that was missing that you needed.
Not contributing it back to the project is a stupid tax because it means that each time you want to upgrade to the latest version of the open source code you have to reapply your fix or feature. This is going to cost you in time and if you don't get it right first time it may introduce further bugs into your system. If you have someone else working on the team and they decide to upgrade the open source project to the latest version they might not know that you had "modded" the OS project. You might not even know that the developer that you replaced on the team applied a patch to it and then you're left scratching your head and trying to work out why the upgraded OS code is not working with your project.
Don't pay the stupid tax. Get your fixes and features back into the open source project as soon as possible and give yourself a pain free upgrade path to further releases of the OS project.