If a person raises his head from sajdah and sees that the imam is in sajdah, in the event that he thinks it is the imam’s first sajdah and he goes into sajdah with the intention of performing it with the imam, but then he realises that actually it was the imam’s second sajdah, in such a case, it will be counted as his second sajdah.
If he thinks it is the imam’s second sajdah and he goes into sajdah, but then he realises that actually it was the imam’s first sajdah, in this case, he must complete the sajdah with the intention of performing it with the imam and then go into sajdah again with the imam. In each case, it is better that he complete the prayer in congregation and perform it again.