A misplaced modifier may change the meaning of a sentence completely or create some sort of ambiguity. That is precisely what happens in the second and the last sentences:
-I only want meat, cheese, and lettuce on my sandwich, please.
-My sister wore her favorite skirt to the dentist's office, which was a red tutu.
The placement of the adverb "only" right before the verb in the first sentence gives us the idea that the speaker is saying something like "I simply want... it's no big deal." If "only" were relocated to occupy the position right before the word "meat", it would make more sense, since the speaker seems to be placing an order. The speaker wants only meat, cheese, and lettuce, nothing else.
As for the second sentence, the modifier is the whole structure "which was a red tutu". Its position in the sentence leads us to believe we'll read some sort of detail about the dentist's office. However, it's actually adding information about the skirt. It would be better if the sentence had been written in the following manner: My sister wore her favorite skirt, which was a red tutu, to the dentist's office.
