Special synchronizers exist for special clock relations such as mesochronous, multi-synchronous and ratiochronous clocks, while variants of N-flip-flop synchronizers are employed when the communicating clocks are asynchronous. N-flip-flop synchronizers are also used in all special cases, at the cost of longer latency than when using specialized synchronizers. The reliability of N-flip-flop synchronizers is expressed by the standard MTBF formula. This paper describes cases of coherent clocks that suffer of a higher failure rate than predicted by the MTBF formula, that formula assumes uniform distribution of data edges across the sampling clock cycle, but coherent clocking leads to drastically different situations. Coherent clocks are defined as derived from a common source, and phase distributions are discussed. The effect of jitter is analyzed, and a new MTBF expression is developed. An optimal condition for maximizing MTBF and a circuit that can adaptively achieve that optimum are described. We show a case study of metastability failure in a real 40nm circuit and describe guidelines used to increase its MTBF based on the rules derived in the paper.