There was the eye poke and slipping, but Stipe also overrated Struve's gound game and never tried to use his wrestling.
At that stage Stipe was mainly a counterpuncher who only hurt people when they came after him, so Struve's size and reach posed some problems for him.
Still, in the end it was a great learning experience and Stipe became a better fighter, the main mistake was probably overrating Struve's ground game and not using his wrestling at all, at the time it was a big upset, Stipe was seen as the best up an coming HW in the UFC and then he got beaten by Struve.
After that most people picked Roy Nelson to beat him and him winning that fight was actually seen as an upset.