The patch introduced two critical changes: 1) Added DelCaptcha call immediately after GetCaptcha in VerifyCaptcha, 2) Added error handling for cache operations. The vulnerability stemmed from the lack of immediate captcha deletion, allowing attackers to observe timing differences between cache hit/miss scenarios. The pre-patch code's execution path duration would differ based on whether the captcha existed in cache (valid key) or not (invalid key), enabling side-channel attacks.