I have enabled Disalbe Parallel Apex Testing to solve UNABLE_TO_LOCK_ROW errors. Let test methods run fast, then I add the
isParallel annotation to existing test classes. But that can still cause UNABLE_TO_LOCK_ROW errors event thought each test classes create test data isolate.
I checked the errors which mostly the following one:
System.DmlException: Insert failed. First exception on row 0; first error: UNABLE_TO_LOCK_ROW, unable to obtain exclusive access to this record: 
Looking back to my code that causes the error by creating a new lead record.
My question is why does create a new lead record that can still lock the database? Is there another solution except removing the
isParallel annotation from the error test class?