One thing I wish I could take a blowtorch to in all spelling resources is the "find the correct spelling in among all these spelling mistakes" activity.
You can go into any remaindered bookshop or newsagent that sells children's literacy activities and find stacks of these spelling mistakes activities, often in the "hothouse your child for NAPLAN" workbooks in the carousels by the door.
Why the people who made up NAPLAN put such questions on the test in the first place, thus encouraging children around the country to practice looking at spelling mistakes, and be exposed to hundreds and hundreds of spelling mistakes that they otherwise would not see, I really have no idea.
Each time a learner sees spelling mistakes, a potential outcome is that they will put them into their memory bank, thereafter being more confused, not less, about how to spell these words.
Even in some quality materials
Spelling mistakes activities are common in trashy spelling activities on the internet, and the sort of workbooks that go straight to remaindered, cheap-as-chips bookshops. Please beware of anything published by the many lay, self-appointed language experts out there, and look for materials written by people who understand sound-letter relationships, language more generally, and the literacy research.
"Correct the spelling mistakes" activities are unfortunately also hidden in otherwise quality materials. I even found a small number in the otherwise great Sound Out Chapter Book workbooks, which I use a lot with older learners.
The first thing I do with new copies of any materials is go through with a red pen crossing out any "find the spelling mistakes" activities, and any other not-great activities, and writing "skip this"on them, so I don't accidentally copy them and provide them to a client.
Why to avoid correct-these-spelling mistakes activities
In very simple terms, we learn written words via two interacting systems. One is the sounding-out one, but often the wrong words in spelling mistakes activities have plausible phoneme-grapheme patterns e.g. tabel, murmer, dore. This means that it's hard for learners' sounding-out mechanisms to tell the difference between correct and incorrect.
The second is the word-recognising system, which sucks in the appearance/structure of words as wholes, and saves them up for the next time we meet this word when reading, or for when we're spelling something tricky and have to ask ourselves "does that look right?".
I really don't approve of showing learners' word recognition systems a whole lot of spelling mistakes. It's bad enough that learners have to look at all the mistakes we can't realistically prevent them from making while they are learning, but why would we provide them with an extra dose of someone else's spelling mistakes?
Unlearning spelling mistakes is hard
I had a client last year with an African name I didn't know, and it was spelt two ways in her file, so I asked her which spelling was correct during our first session, and wrote that on my own file, and in my assessment report.
Later on, I found out that we'd miscommunicated, and I had the spelling wrong. I think she was just saying "yes" to everything that day, as she'd had very little schooling, didn't speak much English and was trying to cope in a mainstream Australian secondary school and please everyone.
Anyway, since I'd learnt her name the wrong way, I then had to unlearn it, and relearn the correct way. I found this surprisingly difficult. Every time I go to write her name now, I have to stop and think "is it AR or ER?" and then apply a little mnemonic to get it right.
Imagine having learnt lots of words the wrong way, and written them like that for quite a while, and then having to try to unlearn and unconfuse yourself. Correcting spelling mistakes is one activity which could easily promote this problem, so please just avoid any such materials, and consistently present learners with correct spellings (and spelling alternatives, where these exist, such as US/UK variations).