Observation should not be used for observance.
Of is superfluous in such phrases as smell of, taste of, feel of.
Off should never be used with of; one or the other is superfluous.
Other. After no other use than, not but.
Ought must never be used in connection with had or did. "You hadn't ought or didn't ought to do it" should be "You ought not to have done it."
Out loud should never be used for aloud.
Panacea is something that cures all diseases, not an effective remedy for one disease.
Partake of should not be used in the sense of eat. It means "to share with others."
Party should never be used for person except in legal documents.
Per should be used in connection with other words of Latin form but not with English words. Per diem, per annum, and the like are correct. Per day or per year are incorrect. It should be a day, or a year.