As all of us know as a general grammatical rule,the verb tense in second clause which comes after a simple past tense clause must be in one of the past tenses.
For example:  
__She knew she had made the right choice.__  
__I touched the baby's head who was crying.__  
But I wondered when I saw this sentence.  
__I heard a little boy waving his hands above the water.__  
__I saw a little boy drop his ice cream__   
Shouldn't it be:  
I heard a little boy was waving his hands above the water?  
I saw a little boy had dropped his ice cream.   
Are there any exceptions to this grammar rule? If yes,what are those specific verbs ?