I don't know that I've ever seen anything strictly _wrong_ in an article about mathematics, but I've certainly seen articles giving equations without defining what any of the variables are and using using very field-specific notation without defining or referencing it, making them impossible to read unless you know enough about the topic to reverse-engineer what they're talking about.

And I'm not talking something like using a Dirac delta without referencing it; I'm talking about pretty esoteric notation that is really only relevant to the topic of the article.

And of course plenty of the math articles are just not very coherently written (and yes, plenty are quite good).