Registrars have done this but it's also 3rd party websites that you search through that will grab domains you search for.

I've only ever had this happen once in the 10+ years of registering domains. It was a few years ago and I submitted an order for a domain with UK2 (then Stargate) for a client. Nothing amazing keyword wise as it was just the client's firstlast.com. Well, I didn't pay the invoice straight away (waited a day) and I go to pay only to have it error on me saying it was already registered.

Was pissed off about it. Looked up the whois for it and saw another individual registered it. Contacted him and he apologized then canceled the domain's registration on his end.. In a few minutes I was able to register it myself. Not sure how his methodology was for snatching up the domain as I didn't run any searches other than on Stargate itself. Either way, I lucked out since he responded right away and was able to cancel it.