Pet Hate
Mixing singular with plural.
"There's" is short for There is.... and should not be used with a plural.
There's many fish in the sea is really saying there is many fish in the sea, which is grammatically incorrect. It should be, there're many fish in the sea which is short for there are many fish in the sea.
Examples of this misuse in speech can, amazingly, often be found on TV news and documentary programmes.