I see that the theory says that if I want to test for difference in mean between 3+ groups I should use ANOVA first, and if ANOVA rejects the null hypothesis then I might want to look into Tukey as a post-hoc test to find which groups' means really differ.
My question then is: why not running Tukey directly? What really happens if I run Tukey without running ANOVA first? Which are the risks/implications of that?
The intuition tells me that if Tukey finds some significant differences, then ANOVA should have found it, too. When is that not the case?