In your training loop you could use something like this:
# works
target = torch.tensor([69, 68])
assert (target<70).all(), "target: {} invalid".format(target)

# fails
target = torch.tensor([69, 70])
assert (target<70).all(), "target: {} invalid".format(target)

which would trigger the errors for target values larger than the used threshold.
Once you are running into these errors, check why you expect them to be <70 and why they are apparently not in this range.