See the Figure below:
The consequence of non-ideal flow in a reactor is a reduction in the residence time.
Stagnant zones can form at the corners of a reactor, reducing the effective volume available for reacting fluid. In short-circuiting, the inlet fluid takes a short-cut route to the outlet, without undergoing conversion in the bulk of the reactor. Similar to short-circuiting is channeling or bypassing in a packed bed, in which the inlet fluid bypasses the catalyst particles to the outlet.