1) The cat explodes
2) You start channeling the Smurfs/Tellytubbies
3) You start to believe that doing the 30th, 29th, 28th and 27th Enochian
Aethyr in the course of one evening will be good for your magical development.
4) New Age stoners tut and say: "Should have done a Lesser Banishing
ritual dude!"
5) You suddenly realize you accidentally mistook Invocation for Evocation,
and there is now a snarling, evoked inner demon standing behind you.
6) The Gods are so embarrassed/angered by the ritual that they smite you
to death with lightning bolts.
7) The walls of your temple/working area start to glow in the dark.
8) The local police force arrive en-masse to arrest you (or, depending on
what kind of ritual you're doing: the local police force don't arrive en-masse
to arrest you.)
9) It just doesn't work. (Probably the most common sign, however there have
been unfortunate individuals who have experienced the others...)