This study identifies and investigates the extra plateau period (EPP), a previously unreported phenomenon in hospital pressure steam sterilization, where temperature and pressure stabilize at elevated levels before entering the sterilization stage. Utilizing digital temperature-pressure detectors and standard test packs, a comprehensive analysis of 58 sterilizers across 31 hospitals was conducted under small-load (Bowie-Dick test) and full-load (sterilization program) conditions, in accordance with the Chinese standard Technical Requirements for Large Steam Sterilizers (GB 8599) or the European standard Sterilization - Steam Sterilizers - Large Sterilizers (EN 285). The findings revealed that 39.66% of the sterilizers (23 out of 58), all of which were older cross-atmospheric pulsating models, exhibited EPP. The Bowie-Dick test program revealed that all affected sterilizers exhibited EPP, with plateau times averaging 462.5 seconds - a 118.88% increase over the established values. A similar trend was observed in 73.91% of sterilization programs, which exhibited EPP, resulting in a 39.91% increase in plateau times (749.9 seconds vs. 536 seconds). The consequences of EPP manifested in substantial delays in temperature penetration within test packs, leading to Bowie-Dick test false negatives and rendering physical monitoring ineffective despite apparent compliance with traditional quality checks. These findings underscore critical risks to sterilization quality, including undetected equipment malfunctions and compromised infection control. The study underscores the necessity for enhanced parameter monitoring in older sterilizers and advocates for precision quality control strategies to ensure patient safety.