During test execution, automated software tests can interfere, i.e., their results can deviate depending on their (possibly interleaved) execution order. Such interference imposes severe restrictions on regression testing, when execution order is not or cannot be controlled for, as they can lead to non-deterministic deviations of test results giving false indication of regressions in the code base. While the phenomenon has been extensively studied for Java and Python projects, it remains unclear if or how the obtained results apply for other languages with different testing practices. Our study contributes to filling that gap by reporting results from a large-scale study on test interference in 134 C projects.