In the episode "A Knight to Remember" Mina went into diary mode instead of Trina because they switched bodies when the crazy comet came. She did this when she found out she would soon be Trina's troll again.
Plot
The entire thing was basically the same, only Mina went into diary mode.
Trivia
Although this was Mina who went into diary mode, she was using Trina's diary rather than her own.
When Mina was in diary mode, only her pupils were glowing red but when she was writing in it from a side view, her entire glasses were glowing red.