There is currently display bug when the first dice in the list is 0d. There is a fix awaiting deployment to fix this, so don't worry about it. It is only a visual thing where it replaces commas with arrows.
If you are too quick clicking the second dice you can end up with two rows of rolls. Wait till the first row has appeared before clicking the next, once the first is there, you can click as fast as you like.
There is no way to automatically remove dice if you click the wrong thing. You can just delete the text from both Reason and Roll fields, or wipe them and start again (there are not that many elements to these pools).