Private blog networks.
These are groups of sites owned by one person or company and built only to pass links. They often look normal at first glance, but they link to each other and to paying customers, not to anything a reader would want.
Link swaps at scale.
Trading one link here and there with a partner you really work with is normal. Setting up dozens of "you link to me, I'll link to you" deals is a pattern.
Bulk or automated links.
Hundreds of directory listings, forum profiles, or blog comments posted in a week don't look like a business getting noticed. They look like software.
Paid links with no label.
When money changes hands for a link, Google expects it to be marked as sponsored. A paid link dressed up as a normal one is exactly what its rules target.
Anchor text matters too. Real links use all sorts of wording: your business name, your web address, "this guide," or a short phrase about your service. If most of your links say the exact same thing, like "best roofer Lincoln NE," that sameness stands out. It's one reason pushing for exact anchor text on every link is a bad idea, even when you could get it.
Speed is another tell. A local business that suddenly picks up 80 new links in a month, with nothing new on its site to explain it, looks odd. A few good links a month from sites that fit your trade looks like what it is: a business people are starting to point to.
So what happens if risky links get caught? Most of the time, Google just ignores them. You paid for nothing, but your site isn't hurt. In worse cases, a person at Google reviews the site and issues a manual action. You'd see a notice in Google Search Console, and parts of your site can drop or vanish from results until you clean it up. Cleanup means finding the bad links, asking site owners to remove them, telling Google to ignore the rest, and then asking for a review. That can take months you didn't plan for.
That's why a cheap batch of links is rarely a bargain. The safe kind costs more per link because someone has to earn the yes. But it's the only kind that keeps adding value instead of sitting there as a risk you'll have to pay to undo later.