Microservices
Individual serverless components have the capability to scale infinitely (up to the capacity of your cloud provider, which is infinite when infinity is considered relative to an average company's compute power). This sounds, and indeed is, amazing and very useful, but these components tend to expose issues with application architectures that you previously couldn't see. This issue is called serverless backpressure. The following backend components can be impacted by this backpressure:
Third-party APIs: It is incredibly common to depend on third-party APIs such as Stripe, Twitter, and Paypal these days. Whether these are to provide payment services (like Stripe or the Bitcoin network) or to provide data (like Bloomberg or Reuters), most applications have an integration of some kind. Serverless applications can cause unexpected problems with these relationships by flooding them with requests far beyond what they can handle. These services often have a rate limit in their SLA too...
