The realization of what just came out of his mouth hits him rather like a bucket of cold water to the face. Possibly with bucket included.

He shakes his head once, already flushing with embarrassment. "I'm sorry," he says in his own voice. "I—forgot I wasn't—I'm sorry."

That may very well be the least coherent eight-word sequence Obadiah has ever heard out of Sherlock Holmes. 'Flustered' is not exactly a common state of being for him.