Red flags can be more difficult to detect when you are using an agency, because some of the important interactions that might give you some clues they are arseholes, are lost if the back-and forth is done by the agency.
Two of the worst red flags were when I was very early in my career, was too naive / a pleaser, and ignored the red flags.
Interview was in the upstairs floor of a warehouse unit. Working for the MD. Job sounded really interesting, lots of autonomy, a chance for real advancement. The boss came across as genuinely a good guy. Trouble was, where I would be working had absolutely no natural daylight, no windows, 100% artificial light and it felt like a prison. I turned the job down, environment is as important as the job to me. The boss (and I, in fairness) was gutted when I declined, but I knew I would be miserable.
Another time, I got through interview 1 and interview 2 was to meet the boss. What a complete arsehole, and I should have walked out. HR said (before I went in to meet Mr AH) "good, I'm glad you've worn heels, Mr AH likes his female employees to be tall and slim, make sure you stand up nice and tall when you go in, won't you". At that point I was already
.
Mr AH's interview technique was shite, (sneering, patronising) "on your CV you say you enjoy reading the Classics, what do you mean?" "Well, I like classic literature...." "No, you haven't answered the question, what do you mean by Classics" Nerves now getting the better of me, mind gone to mush and he knew it "um.... Jane Austin, um ... " "no you still haven't answered my question!! What do you mean by the word Classics". The most humiliating experience I've ever had. He offered me the job, quite a high salary for me at the time. I accepted through the agency, but asked them to find out if there was a pension scheme - there wasn't, but he then withdrew his offer, just because I'd asked. I was gutted, and pleaded with the agency to get him to change his mind. That was a very lucky escape, even though I didn't realise til a lot later how bad it would have been.