I say, and with Allah's success: If one finishes reading Al-Fatiha and contemplates which Surah to read for the duration of performing a pillar while silent without mentioning or glorifying Allah, he must perform the prostration of forgetfulness. Similarly, if he finishes Al-Fatiha and the Surah, and remains silent without bowing for the duration of performing a pillar or contemplates in his prayer without engaging in reading or glorification until he stays silent for the duration of performing a pillar, he must also perform the prostration of forgetfulness. If this is done intentionally, it is called the prostration of intention, and Allah knows best.