1. By mistake.

Nuttall's Standard dictionary of the English language By Nuttall, P.Austin

• mistakely,
• mistakily,
• mistakend,
• mistakelny,
• mistakened,
• mistakeingly,
• mastakenly,
• mistakenley,
• mistankenly,
• mistakedly,
• mistakently,
• mistenly,
• mistakinly,
• mistaknely,
• mistakingly,
• misstakenly,
• mistankely,
• mistakenl,
• mstakenly,
• mistaknly

• mistake,
• mistaking,
• mistaken.

The adjective form of "mistakenly" is "mistaken".

The plural form of the word "mistakenly" is "mistakenlys." However, "mistakenly" is usually used as an adverb and adverbs generally do not have plural forms.

The singular form of the word "mistakenly" is "mistake"