This is an activity I like to do when a test is coming up. I start off by writing a sentence on the blackboard:
Then I tell the students "there are two problems in this sentence" and write a "/2" on the board. I ask the class if they can identify them, and correct the sentence on the blackboard with another color of chalk:
With that completed, I finish the "/2" part by writing "2/2" for 2 correct.
Of course, this sentence is for practicing simple past tense, but you can come up with any example you like to practice whichever grammar point you want the students to practice. The key is to come up with sentences with errors that the students frequently make. Maybe they have trouble getting the pronouns to agree, or commonly spell certain words wrong, or aren't reorganizing words in the question sentence correctly, or forget punctuation. By posing these sentences to them as challenges, you aren't telling any specific students that they're making mistakes - you're just showing them some common pitfalls to avoid.
I come up with four sentences to practice, usually with 2 to 4 mistakes each. I write number one on the blackboard, and ask the students to write the correct sentence on their papers. I show how many errors are in each sentence by writing "/2" or "/3" and so on. You could write all four sentences at once if you wanted to, but I only do one at a time so the students can focus better. I give them about 2 or 3 minutes per sentence and let them talk to their classmates as they figure out what the errors are. Once time is up, I ask the students to identify the corrections for each sentence. If the JTE is helpful, they can explain the mistakes in Japanese.
Hopefully after going through all four sentences, the students have a better idea of what mistakes to avoid, without feeling like a teacher was calling attention to any errors they made in the past.
In the file, Page 1 is if you want to print the papers out on one sheet of A4 paper. Page 2 and 3 are if you want to print it out on half-sized double-sided paper - just slice it across the middle.