I understand your concerns. That never became a crisis for us because of a few reasons. 1. An individual rarely deployed their stuff right before vacation (because of this reason). 2. We had an exhaustive QA process (full regression had to pass before deployment). 3. Everyone on the team was senior and could debug other people's code quickly and easily. 4. Other people's code wasn't crap (see #3). 5. Everyone was familiar as to what the other modules were doing.

Just to note, this wasn't a small system. At least a million lines of code and it was a decision support system. Bugs could theoretically cost lives (at least that's what we told ourselves, but it was kind of a stretch).

This was in a software shop back when we cut and distributed CDs. We had a 3 month release cycle which really enforced #2. I understand this is a lot harder when you are doing 2 week release cycles in Agile or something. With 2 week cycles, full regression for every release kinda goes out the window because there isn't time. (one of the down sides to Agile).