One example of what could shut down a clinical study is an adverse side effect, such as an allergic reaction. I’ve seen one clinical trial put on pause because one participant had a severe allergic reaction to the material being administered and it was thought that the whole group was at high risk but it ended up just being this one particular patient. In this case, it couldn’t have been foreseeable. I have also seen clinical trials become shut down because the placebos did not look significantly different from the active group and people were able to identify which medication was which, causing an immediate bias. In this case, the coordinator of clinical trial material was responsible because proper action should have been taken to make the medication look as identical as possible.