Errors happen in a parallel program when two tasks that are executing simultaneously contain operations that access the same memory location. If one of them writes to the location and the other one reads from it, then the result of the read depends on whether it happens before or after the write. If they both write to the location, then the final value in the location depends on which write happens last.
If your tasks access the same memory locations, then, left to themselves, they will tend to trip over each other. You can solve this by adding synchronization code to make sure the tasks are well-behaved when they access shared memory locations, but synchronization code can be tedious to add and hard to get right, and it is easy to end up with tasks that spend more time doing synchronization than doing work.