Software optimization is constantly a serious concern for developing high-performance systems. To accelerate the workflow execution of a specific functionality, software developers usually define and implement a fast path to speed up the critical and commonly executed functions in the workflow. However, producing a bug-free fast path is nontrivial. Our study on the Linux kernel discloses that a committed fast path can have up to 19 follow-up patches for bug fixing, and most of them are deep semantic bugs, which are difficult to be pinpointed by existing bug-finding tools. In this paper, we present such a new category of software bugs based on our fast-path bug study across various system software including virtual memory manager, file systems, network, and device drivers.We investigate their root causes and identify five error-prone aspects in a fast path: path state, trigger condition, path output, fault handling, and assistant data structure. We find that many of the deep bugs can be prevented by applying static analysis incorporating simple semantic information. We extract a set of rules based on our findings and build a toolkit PALLAS to check fast-path bugs. The evaluation results show that PALLAS can effectively reveal fast-path bugs in a variety of systems including Linux kernel, mobile operating system, software-defined networking system, and web browser.