Spoken language has no spaces between its words. Therefore, one of the major tasks facing listeners of any language is determining from the largely continuous stream of speech where the invisible word boundaries lie. Although English is not a language where the position of a word's stressed syllable is reliably fixed, its lexical stress is nevertheless fixed enough that L1 English listeners initially apply the heuristic that strong syllables mark the first syllable of a new word, attempting alternative resegmentations only when this heuristic fails to identify a viable word string (Cutler & Butterfield, 1992; Cutler & Carter, 1987). Thus, English word stress errors can severely disrupt listener processing. This study uses auditory lexical decision and delayed word identification tasks to test a hypothesized English Word Stress Error Gravity Hierarchy synthesizing previous research that has identified vowel quality (Bond, 1979, 1999; Bond & Small, 1983; Cutler, 2012, 2015) and direction of stress shift (Cutler & Clifton, 1984; Field, 2005) as key predictors for the intelligibility (Munro & Derwing, 1995, 2006) of nonstandard stress pronunciations. Results indicate that English word stress errors, when they introduce concomitant vowel errors, matter – and that the intelligibility impact of any particular lexical stress error can indeed be predicted for both L1 and L2 English listeners by this study’s English Word Stress Error Gravity Hierarchy. These findings have implications for L1 and L2 English pronunciation research, teaching, and testing.