Oh boy! There is an interesting one.
Well apart from checking all the technical skills and blah blah, I always make sure to ask or observe two things about a candidate I am about to interview. And in my opinion someone who lacks this or are weaker in these skills, blows up his/her interview or rather, reduce their chances of getting hired.
As a tester, you should be inquisitive, and that means you have to have some questions when a particular scenario or a test question, is thrown to you.
For example, I asked someone - Here is a dialogue box, and an input field. How would you test it.
When I'm giving you these questions, there are a lot of hidden things and unknowns that as a tester, you should be asking me. If you're the " I'll perform these five/seven functional tests on them" kind, then you are showing me you're quick to jump into testing , even before understanding the context and stakeholders needs. As a tester, you need to understand these vital points because these are pretty important from a client's perspective. As a tester, you also have to understand customers and their needs before launching a flurry of tests.
People who reply with “Can I ask some questions about it first? For instance, what is it for, who will be using it, is this a new feature or an existing one? Knowing this helps me better understand what needs to be tested and how.” are certainly at a better thought process and would be having better chance of getting hired.
Secondly, I have seen people panicking a lot when things don't go or turn out their way. This is an attribute that you should try to find out in a candidate, before hiring. Because, as a tester, you will face these situations a lot when your bug is rejected, or something comes up in Production etc.
If a person/candidate shows signs of panic even for small problems, then chances are that at a higher project level, he/she would not be able to handle the failures which might lead to catastrophic errors/mistakes.
So these two behavioural traits are few red flags that I look for during an interview.