In this study, we examined 139 prospective elementary teachers' solution processes to additive word problems for which the solution is 1 more or 1 less than the answer produced by the straightforward application of the addition or subtraction of the two given numbers. For each problem, five aspects of their solution processes were examined: (a) the modeling strategy or procedure, (b) execution of procedures, (c) the solution, (d) the type of errors, and (e) the implicit or explicit interpretation of the solution produced by the mathematical model or procedure. The major findings of the study were that a majority of prospective elementary teachers' responses (about 91%) contained incorrect solutions to such problems and that about 93% of the errors were +1 or -1 errors. That is, errors due to preservice elementary teachers' failure to interpret correctly the solution produced by the straightforward addition or subtraction of the two numbers given in each word problem. (Contains 2 figures and 3 tables.) [For complete proceedings, see ED500859.]