There’s no running form “rule” that’s always true and no running form “mistake” that doesn’t serve a purpose in the right situation. So even though leaning forward is virtually always the right thing to do (get the details here), there is one circumstance when a distance runner might spontaneously run upright for a good reason.
Share in the comments what you think that situation is and why. Then check back here next week for the correct answer!
And if you’re looking for more reading beyond the forward lean posts I linked to above, check out All Running Form is Good Running Form.