Senior high school students are expected to conduct a research study aligned with the track and strand they are pursuing. This is required under the Department of Education Senior High School curriculum and is a prerequisite to graduation. In this study, common errors in Practical Research 1 manuscripts in terms of content, organization, formality, objectivity, language used, vocabulary, and referencing were identified. The sources of these errors were also determined based on the teachers’ and students perspectives. This study was carried out to determine if there is a significant relationship between the common errors and the sources. A mixed-method research design, particularly explanatory research, was employed in this study. Quantitative data was obtained to determine the common errors where the learners manuscripts were evaluated using a rubric and a researcher-made checklist. Similarly, survey questionnaires were distributed to the teachers and students to determine the sources of errors. A total of 44 schools from the Schools Division of Laguna participated in the study, and purposive sampling was done to identify the respondents. The results of the study revealed that the common errors of the students in their PR 1 manuscripts are: lack of comprehensive information on the problem due to an insufficient number of sources; the texts generally lack transitional devices and logical sequencing; repetitive, misspelled, ambiguous, informal, subjective words and phrases; poor grammar and sentence structure; contracted words and utilization of the third-person point of view; and missing citations in references or bibliography, non-paraphrased citations, or inappropriate citations. While for the sources of these errors, the absence of a research manual, students attitude towards writing research, and their exposure to qualitative studies were causes for carelessness. Lack of exposure to a second language was found to contribute to the first language interference in manuscript writing, and errors in translation were due to the easy access of students to translation tools available on the internet. However, the sources of errors were found to have no significant relationship with most of the common errors in manuscript writing. Nevertheless, based on the thematic analysis, qualitative findings support the quantitative data regarding the common errors and their sources.