This study identified and analyzed the common linguistic errors encountered by Linguistics, Literature, and Advertising Arts majors in their Thesis Proposal classes in the First Semester 2016-2017. The data were the drafts of the thesis proposals of the students from the three different programs. A total of 32 manuscripts were analyzed which was based on the actual number of groups. Results showed that of the three kinds of errors, namely grammatical, syntactical, and mechanics/substance, grammar as a main concern in writing competency was the most common linguistic error among these students. Moreover, the prevalent grammatical errors were: disagreement between the pronoun and antecedent, wrong usage of tense, and disagreement between the verb and subject. In the area of syntax, the most problematic areas were: fragments and run-ons. Lastly, in terms of mechanics, the top errors were: punctuation and spelling. This study recommends that an intensive refresher writing course that focuses on the error-prone areas be conducted to prepare graduating students for their thesis proposal writing; to consider that team teaching and other interventions be considered so linguistic problems together with content can be addressed, since form and content go together, and finally, that a thesis editing guide or writing handbook be prepared, with an abundance of examples, practice exercises and writing activities, for instructors' and students' use.