I perfomed a 2 (groups) x 5 (emotion) repeated measures anova in SPSS. The interaction between groups x emotion is not significant, but Bonferroni post hoc comparisons of this interaction reported a significant difference (p<.0005) between the two groups for each emotion. Is it possible the case in which the interaction is not significant, while post hoc comparisons are significant? There is a mistake?
There is no mistake. Have a look at this explanation:
ANOVA tests the "exploratory value" of a predictor in a model (typically a factor with more than two levels). It answers the questions how likely we can expect a reduction in the residual variance when there was no association of the presumed predictor with the response.
Posthoc tests are apriori unspecified tests, so to say tests after seeing the data and deciding then which comparisons might be interesting. They use pooled variance estimates and control the FWER or the FDR for the family of tests. There are few special cases of posthoc tests that do not efficiently control the FWER. They need a kind of "protection" by a "significant" ANOVA. Most famous example is Fisher's LSD, but this is restricted to 3 groups. Tukey does not require any protection and it keeps the FWER.
In general, ANOVA and posthoc tests answer considerably different questions.
As a rule of thumb, if you are into NHST (null hypothesis significance testing) and didn't have prior expectations (specific contrasts set) you most likely should stop exploring the interaction after discovering it's insignificant.