FPGA input synchronisation
One common technique to ward against potential metastability issues, which can result in cases like this, is to double register your inputs. Like this:


credit: Altera whitepaper


  Metastability is a phenomenon that can cause system failure in digital devices, including FPGAs, when a signal is transferred between circuitry in unrelated or asynchronous clock domains. This paper describes metastability in FPGAs, explains why the phenomenon occurs, and discusses how it can cause design failures.
  The calculated mean time between failures (MTBF) due to metastability indicates whether designers should take steps to reduce the chance of such failures. This paper explains how MTBF is calculated from various design and device parameters, and how both FPGA vendors and designers can increase the MTBF. System reliability can be improved by reducing the chance of metastability failures with design techniques and optimizations.