I pushed out a couple of updates, and included a check for this rare situation, along with a more appropriate error message so as to avoid confusion. Also, if they are looking at the task list and someone else signs up and takes the last spot for a task before they click on the signup list, it will now show the new error message (for all spots filled) and not even show the signup form. But, if they do make it to the signup form and someone manages to take the last spot before they submit the signup form, they will get the same all spots filled error message and the form will not show again (they can then click on the link to take them back to the sheet or tasks).