At least for the nineteenth century, I've seen countless times in manuscripts where corrections were made by simply writing over the existing words, or crossing them out and inserting a little line of text above the crossed-out line with a caret.

Sometimes if they caught their mistake quickly while the ink was still wet, they could wipe it away with a finger and then write over the spot--it usually left a noticeable smudge of ink, but was usually legible.

Sometimes if they caught a small mistake after the ink dried, they would very gently scrape off the top of the paper with a penknife in the spot that needed correction (laid rag paper is considerably thicker than modern paper you put in your printer) and then write in the intended word. Since the action scraped off the sizing off the paper, this resulted in ink that looks a bit spidery as it bleeds into the paper fibers.

If the revised text was lengthy, sometimes they'd cross out the bad text, write new text on a separate piece of paper, and pin or adhere the paper to the book where it was supposed to go with either a straight pin or a little round adhesive wafer that stuck paper together once it was moistened a little or with a blot of hot wax. The specific spot where the insertion was meant to be made in the text was often marked by an added pair of asterisks or some other written symbol.

If a book was in the midst of printing in the days of moveable metal type and a mistake was found that warranted correction, they'd stop the printing momentarily, fix it, and continue printing which resulted in potentially dozens of slight variants within a single printing of a book. Sometimes you'll see errata notices at the end of books or end of newspaper issues alerting readers that they made a mistake that wasn't caught in time to be corrected before printing.

Studying slight variants in text or revised/corrected texts is a huge deal to literary documentary editors or historical documentary editors (meaning, the ones who study and publish texts and not the ones who make filmed documentaries). There are entire books written about the methods of describing texts accurately.

In the Declaration of Independence there is"it's" instead of "its", and one copy of the King James Bible stated "Thou shalt commit adulatory".

hi! in addition to /u/AlfredoEinsteino's response, there was a similar thread a couple of weeks ago - check it out for some more examples

How did the book copiers fix their mistakes?

and another, just for fun

How are ancient stone writings carved so neatly?

Sorry to piggyback, but are there any famous hand written manuscripts or texts that have spelling or grammar errors in them?