Despite an emphasis on experimentation in science curricula worldwide as part of efforts to improve scientific literacy, students encounter challenges, especially with designing unconfounded experiments as part of the control-of-variables strategy (CVS). Becoming aware of experimental design errors is a potential starting point for learners to enhance their experimental skills. Few studies investigated whether learners can accurately assess experiments and are aware of errors. This experimental study, conducted during the COVID-19 pandemic, investigates the accuracy of students’ assessment of experiments and their awareness of self-generated and others’ (vicarious) design errors. 127 students (grade 7–8) were randomly split into two groups. One group conducted an experiment themselves. The other examined an erroneous example of a fictitious peer. Afterwards, both received the same instructions on the CVS and prompts to assess the experiments. Data were collected via worksheets and photos of their experiments. Analysis reveals difficulties controlling all variables in an experiment, especially if they were continuous. Interestingly, while self- and peer assessment accuracy was generally high, students were significantly more aware of vicarious errors than of self-generated ones. This highlights the potential of using assessment of experimental design errors as learning opportunity for experimentation skills, especially when using vicarious errors.