Now, heretek, heretek, heretek.
It's a weird subject, really. The thing is, much like the Ecclesiarchy could be said to be full of people squabbling over theological details, one might say that the Cult Mechanicus are full of people squabbling over technological details (i.e the same damn thing, really). The thing is that the galaxy, hell, even the sector is a damn big place, and there are loads of people with different opinions. The Adeptus Ministorum are not really a very homogenous bunch, so why should the Adeptus Mechanicus be any different? Sure, there are some ground pillars, but each school interpret it differently.
My current GM usually says that what is forbidden is to research new principles of operation. Applying and modifying already researched stuff is fine, like slimming down a shotgun to fit in an arm or making a ridiculously high-powered SP-weapon, as it all relies on tried-and-true mechanics. However, fitting a plasma gun with the above mentioned plasma-spray-thingy would rely on some strange, new device to turn the usual pellet of plasma into a stream.
Some AdMech also believe that the spirit of Omnissiah exist in other things than just machines, placing him/her/it into physical phenomena or the bodies of animals and creatures. Yet others believe that since the Omnissiah exist in all things, even filthy xenotech, it can always be purged and returned to its most holy form.
For example, the "Solo" Boltgun is deemed heretical by some of the Mechanicus, and yet it was probably developed by loyalist tech-priests and manufactured on a tech-priest controlled forge world, or at least at a Mechanicus facility.
In our current game I play as an Explorator declared dead on paper, due to the loss of the ship he served on. As such, the AdMech are sort of iffy having him around (I mean, you can't assign a job to someone who's dead, right?), and spends most of his time annoying the crew of Anastasia with mostly pointless science. But I digress. The thing is, we recently discovered a rather funky crystal, who actually is a plant of sorts. When impacted, it shatters into razor-sharp shards rather powerfully. As such, the sadistic bastard to tech-priest designed a bolter shell around them. This, however, is according to our definition NOT tech-heresy, as it does not rely on any new principles. It's like burning wood. You don't care if the wood is sort of terran or purple and mushroom-like. You just burn the stuff as long as you don't get high and it isn't from the warp.
Of cource, meddling with the warp and psychic stuff is strictly forbidden, but done anyway. We do love dem force weapons, don't we?
And, naturally, knowing the high-ups helps the Arbites look the other way. Just don't expect any help if the Inquisition is after ya.