What's a negative word for "subtle"?

I'm trying to describe software bugs that are hard to track down due to the mistake (introduced by the developer) being a very subtle one.

"when these errors manifest they do so in a ...(pernicious?)... way"

It's on the tip-of-my-tongue and is a word most people have heard of (so it can be used in marketing material without a fear the reader will need to lookup the word).

Insidious is probably your best bet,

adjective
proceeding in a gradual, subtle way, but with very harmful effects.

(Google)

but you might also consider treacherous, which has the same connotation of remaining hidden and doing damage unexpectedly:

adjective
1. guilty of or involving betrayal or deception.
2. (of ground, water, conditions, etc.) presenting hidden or unpredictable dangers.

(Google)

Elusive

adjective
difficult to find, catch, or achieve.

(oxforddictionaries.com)

Or, more fun and more negative:

Sneaky

adjective
1. furtive; sly.

(oxforddictionaries.com)

Obscure is a possibility

hard to perceive

not readily seen

inconspicuous or unnoticeable

However, this would work better if it's a bug that's difficult to reproduce, rather than a bug which is difficult to find the cause of.

Ref: dictionary.reference.com

pernicious

One adjective that comes to mind is " pernicious ".

Pernicious : having a harmful effect, especially in a harmful or subtle way.

Perfidious

Surprised no one has mentioned perfidious.

adjective 1. deliberately faithless; treacherous; deceitful: a perfidious lover.

Treachery implies subtlety.