Field experiments are now a major tool in the repertoire of ecologists to study populations, resources and interspecific interactions. Notwithstanding a general improvement in the sophistication of experimental work, there remains a major problem when manipulations involve the forced confinement of mobile organisms in enclosures (or ‘cages’). Some evidence suggests that outcomes of field experiments depend upon the size of the enclosure used. Simulated field experiments are conducted here to explore some of the implications of the effects of enclosure size on experiments involving consumers engaged in exploitative competition for food resources. The simulations employ calibration data from rocky intertidal systems of South-Eastern Australia, but they are not designed to imitate a specific system but rather to illustrate some potentially general problems associated with confinement experiments. The simulations indicate that there will be enclosure size-dependent effects on consumption rates of individual, with the severity of effects being inversely related to the enclosure size used (i.e., worse artefacts for smaller enclosures). The mechanism underlying the effect appears to be the more concentrated harvesting of resources by consumers confined to small enclosures, which prevent the food resource from ‘escaping’ predation as frequently as occurs under unconstrained conditions. The simulations are extended to investigate how the interpretation of interspecific competition may be influenced by the size of enclosure used and, if the results reflect natural conditions, then misleading interpretations are likely to emerge if enclosure sizes are too small relative to natural scales of movement.