Execution tracing has an observer effect: the act of tracing perturbs program behaviour via its overhead, which can in turn affect the accuracy of subsequent dynamic analysis. We investigate this observer effect in the context of concurrent behaviour within JVM-based programs. Concurrent behaviour is especially fragile as task-scheduling ordering can change, which could even lead to deadlock via thread starvation under certain conditions. We analyse three dimensions of overhead, compute volume, memory volume, and uniformity, using a configurable-overhead tracer and a concurrency-performance analyser. We argue that uniformity is a key, and underappreciated, dimension of overhead that can have qualitative effects on program behaviour. Experimental results show that overhead significantly affects real-world concurrent behaviour and subsequent analysis, at times unintuitively.