Are we talking about "breaking the rules" as in not correctly reporting your current HP, or "Breaking the rules" as in "I think my spring boots should allow me to jump further than indicated by the rules", or "breaking the rules" as in "We got a magical crossbow and a bunch of peasants, let us make a railgun"? Or maybe some alternative I can not think of right now?
In my case, I'm more talking about the bolded. There's a difference, I think, between outright cheating (misreporting die rolls, reading the module the DM's running, etc.) which is always bad, and pushing the envelope looking for exploits (which IMO it's then the DM's job to shut down) or cool interactions between different effects or even just trying things the rules don't cover.
"Let's see if, with some creative use of
Stone Shape, I can make this whole side of the BBEG's castle collapse" is the sort of thing I mean. That's not an intended use of the spell but until-unless the DM tells me I can't, there's nothing to stop me from trying it.
"I'll disable that enemy ship by flying over and casting
Shrink on its rudder" is another example, one our DM let us get away with.
"By your description I must outweigh this guy by 100 pounds easy, so instead of attcking with my sword I'll just bodycheck him off the cliff" in a game that doesn't have rules for such maneuvers.
More generally, it's the slightly Chaotic attitude of "think outside the box and then just do it until something tells you to stop" rather than the Lawful "stay in my lane and wait for something to give me permission". Push the envelope rather than submit to its constraints.