Recaptcha comes to mind. That said, there are quite a few services which battle these systems for you nowadays (such as scraperapi - not affiliated, not a user

Recaptcha comes to mind. That said, there are quite a few services which battle these systems for you nowadays (such as scraperapi - not affiliated, not a user). They are not always successful, but they have an advantage of maaany residential proxies (no doubt totally ethically obtained /s, but that's another story).