The ability to solve equations and inequalities is necessary for success in algebra. However, reasoning biases and misconceptions may create barriers for students to build knowledge of algebraic symbols and their values. This study investigated whether students' errors when solving equations and inequalities could be attributed to their tendency to misinterpret the phenomenal sign of an expression (e.g., -2x interpreted as representing negative numbers only). Additionally, the study examined whether an intervention using erroneous reasoning examples in refutational texts would be more effective than correct examples in helping students address the specific misconception. The study involved 119 9th-grade Greek students who underwent Pre-, Post-, and Retention tests. The Experimental Group (N = 44) saw erroneous examples of reasoning with solving inequalities in refutational text, while the Control Group (N = 65) saw correct examples in non-refutational text. The results showed that students' misinterpretation of the phenomenal sign in algebraic expressions may influence their mistakes when solving certain kinds of equations and inequalities. Both erroneous and correct examples were effective in helping students address some of their misconceptions, although the gains were not sustained in the long term.