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.