Is this correct ? Can we use two different tenses , like this one for example : ( happened and drops )
“What my parents think happened when a bottle of shampoo drops on the bathroom floor.....” (then it continues with a meme).
I thought it would be better if we say:
“what my parents think happened when a bottle of shampoo dropped on the bathroom floor ...”
Or
“what my parents think happens when a bottle of shampoo drops on the bathroom floor...”
Indeed, you can't mix two tenses like that;
"what my parents think happens, when a bottle of shampoo drops on the bathroom floor"
is the correct way to phrase this, unless you're talking about something that has happened, in which case
"what my parents think happened when a bottle of shampoo dropped on the bathroom floor"
would be correct.
In your particular case, the two examples you gave are incorrect and you do need to match tenses (as you thought and as Ben said in his answer).
But for the more general question, you can certainly mix tenses in a sentence, e.g.
I was going to the store when I remembered that I had left the water running, so I rushed back home to shut it off.
"Was going" = past continuous "Remembered" = past "had left" = past perfect "water running" = present continuous "rushed" = past "to shut" = infinitive
