One problem that can occur with informal testing is cheating. Students sometimes take advantage of informal test types such as teacher-created worksheets or take-home quizzes. Even when given orders not to, some students work together or copy from one another. This is a downfall of informal testing, as a teacher cannot accurately assess each student's personal knowledge.
When teachers create their own tests, they are trying to get an idea of what students have been learning in their class. However, the material that is present on the informal tests may not always be what other students in that particular academic level are learning. While curriculum may be similar, it's not always the same for students in differing math classes within the same academic level and school. This can cause some students to either be behind others of their academic level or clueless when it comes to certain aspects of the curriculum. This is difficult for teachers to remedy, as often their required course load is more than they are able to fit into one semester.
In creating their own worksheets, teachers add an extra element of practice that the textbook may not offer. While this type of testing is beneficial for many students, some students are inevitably left behind. Many teachers are forced to move on to a new topic when the majority of their students have reached an understanding of the lesson. When this happens, those that have not mastered the material are usually passed over unless they reach out to the teacher.
Many math teachers give students criterion tests throughout a semester. Criterion tests contain an all-encompassing amount of subject matter, usually from a quarter or for the entire semester, and are usually created by the teacher. One downfall of this type of informal testing is that by the end of the semester, students may start to confuse subject matter when they are given an all-encompassing test. It's important to provide students with a proper review to help make sure that this problem doesn't occur.