How common were misspelling and other writing errors in historical documents?
I am aware that the complimentary goods used to create texts were expensive at some points in history especially before the printing press.
Also the way english was spelled in the 18th century, any explaination?
I want to separate this into two questions: (1) are there mistakes in handwritten documents? and (2) do premodern documents have spelling mistakes?
For (1), the answer is decidedly yes. Anyone who copies a large volume of text will occasionally make a mistake. The most common mistakes are things like a common word substituted for a similar-looking rare one (often because the scribe didn't know the word so they thought the text they were copying from must have a mistake in it), or skipping a line or two if several lines begin with the same word. Basically the same kinds of mistakes people sometimes make when they read aloud. Good scribes messed up much less frequently because they were highly trained, but often we have documents copied over by students or other people without a lot of training and it shows.
(2) Did they make spelling mistakes? The whole concept of standardized spelling in English is pretty new, so they often spelled things in a way that your teacher would mark wrong if you wrote it now, but they weren't wrong because those rules hadn't been invented. Of course they also sometimes did the kind of misspelling where you just randomly write the wrong letter by mistake or forget a letter or something, but that's different.
Hi OP, if you could be a little more specific, that could greatly assist anyone considering answering here. Primary source documents from where/when? Can you provide some examples of the 18th c spelling that you're wondering about? Thanks!