Background: Trigonometry is an area of mathematics that students believe to be particularly difficult and abstract compared with the other subjects of mathematics. Trigonometry is often introduced early in year 8 with most textbooks traditionally starting with naming sides of right-angled triangles. Students need to see and understand why their learning of trigonometry matters. Aims: In this study, particular types of errors, underlying misconceptions, and obstacles that occur in trigonometry lessons are described. Sample: 140 tenth grade high-school students participated in the study. 6 tenth grade mathematics teachers were observed. Method: A diagnostic test that consists of seven trigonometric questions was prepared and carried out. The students' responses to the test were analyzed and categorized. Observations notes were considered. Results: The most common errors that the students made in questions were selected. Several problematic areas have been identified such as improper use of equation, order of operations, and value and place of sin, cosine, misused data, misinterpreted language, logically invalid inference, distorted definition, and technical mechanical errors. This paper gives some valuable suggestion (Possible treatment of students' error obstacles, and misconceptions) in trigonometric teaching for frontline teachers. Conclusion: The study found students have errors, misconceptions, and obstacles in trigonometry lessons. (Contains 2 tables.)