What problems can arise when parallelizing testing?

Test instability due to dependency on execution order or overall state. The need to manage resources (processors, memory, browsers, test data). Problems with synchronization access to shared data or external systems. Difficulties debugging test failures, reproducing issues in parallel mode can be harder. Increased complexity of configuring and maintaining the test environment. Logging and reporting may become less clear when running in parallel. Possibility of false positives due to race conditions.