To reproduce the bug:
1.create a new course or use an existing one
2.add item 'choice' with defaults values excpet the following settings:
Limit the number of responses allowed = enable
some choices where limit = 1
Restrict answering to this time period = yes
Publish results = always show results to students
privacy of results = publish full results, showing names and their choices
group mode = separate groups
3.login at 2 different PCs with different student account
4.select same choice and hit 'save my choice' synchronously
As you can see in the attachment, choosing a limited choice is possible. We have received some complaints by teachers using this tool. They report that (somehow) this effect happens quite often, especially when tight time period is enabled.
Thank you for your help in advance!!!