It comes to my mind that while some problems have obviously wrong solutions, they're still accepted in an algorithmic contest. E.g. https://mirror.codeforces.com/blog/entry/147991, or (possibly) https://qoj.ac/contest/1901/problem/8614, https://dmoj.ca/problem/coci16c4p4. Would this be regarded as an error, or actually acceptable? (Output only, or partial scoring problems are excluded from this).

A common "excuse" is to generate weak tests to deliberately make the intended etc. solutions pass.