All the time. Causes include:

• Test depends on an external system (database, package manager)
• Race conditions
• Failing the test cleared bad state (test expects test data not to be in the system and clears it when it exits)
• Failing test set up unknown prerequisite (Build 2 tests depends on changes in Build 1 but build system built them out of order)
• External forces messing with the test runner (test machine going to sleep or running out of resources)

We call those "flaky tests" and only fail a build if a given test cannot pass after 2 retries. (We also flag the test runs for manual review)