ספר דברים by nature is full of challenges relating to the tense of verbs. There was one that slipped by me a few years ago. It's not that I didn't catch it, just that I failed to correct it on the spot after deliberating in my mind. Not sure which is worse.
משה רבינו relates (א:י"ג) that he instructed the nation to gather wise men וַאֲשִׂימֵם בְּרָאשֵׁיכֶם. Although this is being related in the past, he is stating that he said in the past that he will place these men as heads of the nation - in the future. The בעל קריאה mistakenly put a קמץ under the וי"ו of ואשימם which I am pretty sure would change it from future to past. This is an easy mistake to make as everything which follows is indeed in the past tense. This is also tricky to catch since the difference between the וי"ו ההיפוך and the regular וי"ו is a שוא and a קמץ or פתח. But here it ends up being a difference between a קמץ and פתח. (See the comment by Bezlalel for more detail as to why this ends up being more tricky.)
Hopefully recording this now will help me be more mindful of it in future years.