Past Tense - S1-E4
Plot hole: Meredith submits to the drug test at her office. In the next scene, which takes place later the same evening, Harrison tells Tru that Meredith's test came back positive and she was immediately fired. But it takes at least 24 hours to get a result from a urine test at all. If the test is positive, the sample must be subjected to a more thorough examination. It usually takes at least 72 hours to get the results in that case. There's no way Meredith's positive results could have come back in less than six hours, even if a lab had been open late in the evening.