Giant bladder stones, defined as stones weighing over 100 g and/or measuring more than 4 cm in diameter, are relatively uncommon compared to other types of urinary tract stones. This patient, an 85-year-old male with an unknown medical history, initially presented with urinary incontinence and hematuria. Radiological findings revealed a large prostate, a forgotten left renal double-J (DJ) stent for more than 20 years with an encrusted bladder stone, and additional calculi in the lower pole of the left kidney. The patient underwent laser cystolithotripsy, but due to the complexity of the case, a second procedure was scheduled. Following the second procedure, the patient experienced a generalized tonic-clonic seizure and subsequent loss of consciousness, which was attributed to hyponatremia. The patient received appropriate management to correct hyponatremia and antiepileptic medication to control the seizure. The patient's condition eventually improved and he was discharged home with prescribed medications and follow-up appointments. This case emphasizes the potential complications of giant bladder stones and a forgotten DJ stent in an 85-year-old male patient as a rare consequence following such a rare presentation.