Though people that have authority over important projects should have proper security, considering how large the internet is, with how many individual parts, the chance of someone being in charge of a large and important project - may it be a browser, compiler/interpreter, utility, library etc. is not even close to zero.
So if a (co-)maintainer of a project included as standard utility in Linux Servers, let's say bash for example, is somehow breached, the attacker could push and force merge a malicious obfuscated commit, maybe even with normal content included. As it's from a reputable source, it's not going to be checked as thoroughly as commits from other people. One hour later, every Arch system, desktop and server, has a trojan. Four hours later also all Gentoo systems (got to compile it first). 2 ~~years~~ weeks later regularly updated debian servers now contain malware. A chain of events, fragile to being detected by people monitoring their own activity, other maintainers activity and people reading the source - eg. for security reasons -, but yet, not that unlikely considering the amount of packages present even in a standard install, and needed as dependencies for typical server packages.