In a Representative Line article dated 2026-09-29, Remy Porter describes a flaky CI/CD build on Nikolai M's team. One test failed in roughly 0.5% of runs. That was frequent enough to be annoying, but not frequent enough to prompt management to assign an investigation. Nikolai eventually examined the failure himself.

test.cs
Assert.DoesNotContain("000", transactionId.ToString());

The test called Assert.DoesNotContain("000", transactionId.ToString()). The transactionId value was a GUID. The assertion tried to identify an empty UUID by assuming that three consecutive zeroes could not appear in a normal string representation. Nikolai measured the assumption and found the sequence in 0.5527% of cases, or about one in 181.

The article notes that Microsoft's GUID implementation is documented as a wrapper around UUID algorithms and supports several UUID variants. Its practical recommendation is to compare the value with GUID.Empty, the built-in constant representing a UUID with every bit set to zero. An explicit empty-GUID check expresses the intended test directly.