Wire.available() and Serial.available() : How do they differ?

Projects Networking, Protocols, and Devices
4

If I have understood Post#1 and Post#2 correctly:

1. When the function Wire.requestFrom() executes correctly, the value returned by this function will always be equal to the value returned by Wire.available() function.

2. If the function requestFrom() is executed partially due to internal fault either in the Master or Slave, the value returned by requestFrom() will be less than the value returned by available(). What value will Wire.available() return in this abnormal case?

Thanks to pylon and cattledog for the valuable information/comments.