Profiling and benchmarking are separate but complementary tools in the developer's Swiss Army knife and anything can just gone wrong twice with the many optimisations that a modern JVM can do. "FlameGraph and JMH (Java Microbenchmark Harness) are sharp razors that need to be carefully handled to void being cut by: during the intervention a "peek under the hood" will be given to some JVM mechanisms to understand the reasons behind the failures of most well known profilers and will be presented a journey among (negative) benchmarking examples to learn how to do them right.