This case from New Jersey illustrates the border line between behavior that is so bad that an expert is not needed at trial to testify on whether there was a deviation, and behavior that is bad, but still requires an expert.  The NJ court found in TARUTIS, v. ALAN ACKERMAN and GREGORY WISOTSKY,  that:

"No expert is needed "where the questioned conduct presents such an obvious breach of an equally obvious professional norm that the fact-finder could resolve the dispute based on its own ordinary knowledge and experience and without resort to technical or esoteric information." Brach Eichler, supra, 345 N.J. Super. at 12. For example, no expert testimony was required when the attorney entirely failed to submit a legal argument in his client’s defense, Sommers v. McKinney, 287 N.J. Super. 1, 8-12 (App. Div. 1996), or where the attorney failed to comply with the controlling statute of limitations, Brizak v. Needle, 239 N.J. Super. 415, 429, 431-33 (App. Div.), certif. denied, 122 N.J. 164 (1990). "

But, where the defendant attorney  "did not retain an expert to testify to the coefficient of friction of the floor, nor did he propound interrogatories, depose any witnesses, request documents, or request admissions from Resorts" the legal malpractice client still needs an expert to testify that the attorney was negligent.