Manuscripts allows you to check your spelling and grammar as you type to avoid disasters like above. Grammar and spellchecking can be accessed from Edit > Spelling and Grammar in the main menu.
Check spelling and grammar manually
To check spelling and grammar manually, choose Edit >Spelling and Grammar > Show Spelling and Grammar (⌘:).
To update the spelling and grammar analysis of your document after you have made some changes, choose Edit >Spelling and Grammar > Check Document Now (⌘;).
Check spelling and grammar as you type
Spelling and grammar can also be checked as you type, by toggling on Edit > Spelling and Grammar > Check Spelling While Typing. Grammatical errors are presented in the familiar green, and spelling errors in red.
Correct Spelling Automatically
You can even make Manuscripts correct your spelling for you by choosing Edit > Spelling and Grammar > Correct Spelling Automatically.