In a sample of 10 professionals interpreting the same source speech in the simultaneous mode, errors and omissions (e/o’s) were found to affect different source-speech segments, and a large proportion among them were only made by a small proportion of the subjects. In a repeat performance, there were some new e/o’s in the second version when the same interpreters had interpreted the same segments correctly in the first version. These findings strengthen the Effort Models’ “tightrope hypothesis” that many e/o’s are due not to the intrinsic difficulty of the corresponding source-speech segments, but to the interpreters working close to processing capacity saturation, which makes them vulnerable to even small variations in the available processing capacity for each interpreting component.