The functional safety is the part of the overall safety relating to equipment/ system/ installation and their control systems that depends on the correct functioning of the safety-related systems. Due to the critical role of safety-related systems for managing risks, international standards have been developed to provide guidelines and requirements for all their safety lifecycle activities. The IEC 61508 and IEC 61511 are now recognized all around the world and have become the references for the best practice of functional safety. However, a decade of on-the-filed functional safety experience has shown that several concepts used in these standards are still subject to common mistakes in their interpretation or implementation, which may result in significant loss of safety. This paper proposes a review of ten common mistakes in functional safety, relating to: Safety Lifecycle (SLC), Functional Safety Management (FSM), Safety Integrity Level (SIL), Safety Requirement Specification (SRS), PFDavg and PFH, System architecture “M-out-of-N” (MooN), “Safe Failure Fraction” (SFF), and Certification. This review aims at contributing to a better practice of functional safety.