Opennmt-py demo training failed from time to time

[pytorch_user]

I used below command to train a demo model.
python train.py -data data/demo -save_model demo-model -gpu_ranks 0 -tensorboard

But the training failed from time to time(see below tensorboard, there are two runs. One is success with high accuracy, the other one is not. ). Is the result expected?

thanks in advanced.