Unwanted tooth movements caused by long-term bonded fixed retainer placement or improperly placed orthodontic appliances can cause adverse occlusal and periodontal effects. This case series reports the treatment of severe complications of long-term fixed retention in two previously orthodontically treated adult patients. Both patients wore a mandibular canine-to-canine bonded retainer, with no evident signs of debonding or breakage; however, the mandibular incisor, which had remained bonded to the retainer, was bearing the entire incisor load, resulting in extreme lingual movement of the root, and ultimately severe lingual fenestration. In both patients the treatment opted for was carefully designed customized appliances comprising clear aligners in one case and a fully customized lingual fixed appliance in the other, to recover proper alignment, with labial repositioning of the displaced roots. The treatment outcomes were satisfactory, and lingual gingival recession appeared resolved in both patients, as confirmed by pre- and post-treatment CBCT. The treatment and clinical manifestations initially observed are discussed, along with the possible causes and considerations for the prevention of similar clinical scenarios.