I've seen so many shady tricks pulled by recruiters. I'm sure it goes both ways, but never forget the recruiter isn't in the business of helping you, they're in the business of helping the people who pay them.
Seen recruiters low-ball staff, or tell the person that they weren't as good as other candidates... "but if you lower your day rate to be more aligned with your junior-level skills..." So when the person shows up, they feel deflated since they think we thought they were junior... but in fact, we loved them and just didn't have enough budget to hire them at the right rate -- and the recruiter helped us get their rates down because at the end of the day the recruiter only cared about putting seats in chairs for us.
Seen recruiters spam over candidates without so much as doing a basic screening interview. "Oh yeah, he's great... he knows JavaScript and English..." and they're literally just looking at the poor guy's LinkedIn and they haven't ever spoken with him past a few copy-paste emails. It's a numbers game to them. They don't want you, as the person paying them, to ever feel like their shelves are empty.
Seen recruiters promise people visas along with the offer letter... then for whatever reason, if the job shifts, the recruiters just cancel the contract and the person would get deported. Saw this in Sydney A LOT. The recruiters and staffing agencies don't care at all what happens to the person, as long as they get a commission. They lie and over-promise, and sell-sell-sell... and even if they only have a 3-month contract they'll promise someone a year, then just switch it last minute or have a cancellation clause.
As someone who worked for a Digital Agency where we hired a lot of people through recruiters... the number of times some poor bloke would come up to me and be like, "So... 3 months probation then I'm full time? That's what the recruiter said... now you can get me a visa and I'll be able to bring my wife over here too?" and I'd have to be like, "Yeah sorry, Johnny... this was just a 3-month gig." Had one guy, "But I gave up my family's visa for this... the recruiter promised me higher pay and that you'd take over my visa..." Felt awful. And the poor guy almost certainly had to leave Sydney when the job was done.
Worse... my GM wouldn't let me fire that recruiter. "They give us the best rates..." Was all so shady. Left me with the solid impression that these people were all just bottom feeders. Willing to do anything to make a buck that day.