Solving the problem of increasing English language output has been the focus of attention in the last decade. While previous research has extensively analyzed two sources of errors, namely the interlingual and intralingual sources, found in spoken language, this qualitative study investigates the teacher's language as a source of errors for the learners. It analyzes the common grammatical errors committed by 30 Saudi teachers in their spoken English during one-to-one interviews. Error Analysis (EA), linguistic-based classification and linguistic taxonomy of the data reveal seven types of grammatical errors: the wrong use of tenses, errors in the use of prepositions, wrong use of prepositions, errors in the use of articles, omission of a/an, wrong use of articles, and subject-verb inversion in wh-questions, errors due to lack of concord and agreement, and typical Arabic constructions. The most dominant errors are due to concord or agreement and the least is errors in the use of prepositions.