Yes. It really is a failure mode. No, it is not fine to stop whenever you want during a test. It doesn't matter what statistical test you are using -- you need to gather enough data to have sufficient statistical power.
Also, the more tests you do the lower the p-value should be.
If you stop when you reach your p-value then you are misinterpreting the results if you claim the results mean anything.
Also, the more tests you do the lower the p-value should be.
If you stop when you reach your p-value then you are misinterpreting the results if you claim the results mean anything.