The study investigates the impact of contamination and improper assembly on the stability of modular hip stem connections in revision surgeries. Modular components offer flexibility in fitting implants to the patient's bone structure but are vulnerable to relative motion, which can cause fretting corrosion and implant failure. The research tested 48 neck-stem connections under various conditions of contamination (native, contaminated, cleaned) and assembly (secured, pre-tensioned). Results showed that contamination, particularly combined with improper assembly, significantly increased neck rotation (35.3° vs. 2.4°), micromotion (67.8 μm vs. 5.1 μm), and axial displacement (34.1 μm vs. 4.3 μm). A significant reduction in mechanical stability was observed in improperly secured connections. Proper cleaning with a new instrument and pre-tensioning of components reduced these adverse effects. The study highlights the importance of thorough cleaning and correct assembly to prevent early implant failure and potential complications.