When counting, especially when rechecking a known or expected total, skip a number to create a 'wrong' answer. Look puzzled with the result, start again. Repeat as required and desired.
This technique can give children a reason to count. I most often use it with younger children when checking the totals in find hte difference activities. Typically, we have two pictures and put down a counter on each picture where there is a difference. There are usually 12 differences. When they have been found I count up the counters correctly on one sheet but then count up starting from 2 with the other sheet. The children I work with usually don't notice this. It leads to a situation where they have seen me count the counters, they can see the counters are the same on both sheets but the number of the count is different. Usually, after doing a miscount like this twice the children will start counting. At that point I stop miscounting so they can verify the correct answer for themselves.
Another use for miscounting is when counting up the number of flashcards, especially in activities where the group end up with a set of cards they have won and the teacher ends up with a set of cards they have lost, for example, a vocabulary review game. In such cases we often compare piles by counting and discarding cards together to see who has more. Inevitably, the children will have more but it is possible to sow confusion and increase attention by pretending to discard cards but not actually doing so. I think it is important that such cheating is always discovered and seen as a blatent. It should be encountered as a joke.