I corrected my kindergarten teacher over a "Whose Lost Teeth?" bulletin board and she never really forgave me. (I'm guessing I didn't do it with much tact). I do think there's a difference between true typos - missin letters, incorrect punctuation! and the like - and errors like "pedals" where the mistake is more likely to be in the knowledge than in the transcription. The former are one-off mistakes; the latter are more likely to be repeated (and possibly taught in place of the correct alternatives).
I would think that most teachers would realize that whenever one is making a poster, mistakes are more likely than not. And its important to get another pair of eyes on it.
It's because your brain is focusing on putting the letters on the poster, spacing them out, making them big enough. The focus is more art than spelling. It's pretty common to make mistakes doing that.
Then again, sometimes people just don't know how to spell.
My mother once helped out in the kindergarten in my elementary school. I was in 2nd grade at the time. Don't know why she was helping out there, maybe they needed the help?
The teacher had misspelled the word "calendar." She had written "er" instead of "ar."
Now, my mother was not a native English speaker but she could read and write perfect English. This did not sit well with her. And while she said nothing to anyone at the school, I think I heard her recount it for at least 20 years or more.