Short description of error
Most blender operators are executed when LMB is pressed or released(always like inset or if 'Release confirms' option is selected in user preferences like transform).
Also, most can be executed by pressing spacebar instead of LMB.(like extrude)
That is not the case with bevel operator though.
(also it looks like inset operator cannot be executed with spacebar as well, it do work on LMB release though)
Exact steps for others to reproduce the error
Use bevel operator then try to confirm it with spacebar instead of LMB