I'm looking for a German saying that embodies the idea that if you check something (a program, a device, a work etc.) and you find one obvious mistake/problem, then further mistakes/problems/errors will follow.
Something like Where there's smoke, there's fire. Or The fish rots from the head. But these are not really right in this context.
The concept is so common, there must be something.
Since it came up in comments, I'll explain why I think this concept is common:
There're solid logical reasons why there would be correlations in terms of error frequency between different programs. Say, two programmers write an app. One is good, one is bad. You test both blindly. If you find one mistake quickly, you'll have a high chance of testing the bad one, which means more mistakes are going to come along. This is basic Bayesian thinking. A fun manifestation is the German tank problem