Hi Kate,
This is because your “pair” column has a trailing space character at the end, which makes all the references to the pair column in the code return undefined. As a result, every single trial generated by it is labeled "experiment_undefined" and therefore the while loop does find three successive trials in new_targets that share a label forever no matter how many times they are shuffled (because they are all labeled "experiment_undefined") and so the script gets stuck there. Just delete the trailing space and it should work
Jeremy