I would like to know what “illegal” techniques Moccers around here have used/avoided using. Modifying parts also count.
I have cut lightsaber rods, axle rods, and occasionally wedged pieces in holes that wouldn’t be traditionally used to place things into. I have also put things under CCBS shells to fill in gaps, which makes the shell bulge out slightly, stressing the joint.
I love putting parts with clips on them on the pistons from G1 parts (Metru torsos, Metru legs, etc.)
Heck, my Vardas MOC from a while back has a crazy amount of illegal techniques:
I see people paint pieces all the time, which I count as an illegal building technique. This is particularly used in the BIONICLE community for Kanohi Masks.
I find it tacky when someone uses one of those hero factory logo pieces on a moc that isn’t hero factory
Same, but half of my clips are cracked as a result, being too lose for any good use now. 
But is that illegal?
I’m gonna call the cops.
You need permits for this.
See, that’s why it’s an “illegal” technique.
“Illegal” techniques are any technique used to make pieces stay together that isn’t a standard connection, like a pin, axle, or balljoint/cup connection.
Modding parts is also an “illegal” technique, arguably so is using non-LEGO parts.
To add to what @Onyx_ said, this PDF from veteran LEGO designer Jamie Berard should help define things:
How about Mega Bloks parts? 
My exo-force arm and skull spider legs breaked while doing the techs. 
This is actually very helpful. I didn’t know some of this stuff!
I have a very strict policy an illegal connections.
I don’t use them.
Does that make it harder for you to make your bio-former MOCs?
Well some times.
But most illegal connections aren’t very useful to me.
For example when the Avengers hellicarrier legalized clip to tile connections I got pretty excited. But I still haven’t used it once.
Using a '02 Krana in one of the “agori pilot mask”.
I’m not sure what it’s REALLY called, but it came in two sets in 2009 (dark red and silver) and in 2011 as a hero factory helmet (in blue). Go ahead, try it.
I still don’t know what piece you’re talking about…
The helmet used for Crotesius, Kirbraz, and Scodonius.
Yeah, the 2010 is what threw me off.
