That looks like rust. It's come up in my neps before, and it can spread between plants. In most other genera of plants, rust appears as brown dust-like rashes on the leaves. However, the species of rust fungus that is genera-specific to nepenthes produces that distinctive orange/copper/red rash.
I find that if the plant is strong and healthy, and the environment is good, this rust is hardly an issue and remains restricted to the oldest leaves on the plant. In this case it is mostly a cosmetic issue.
However, if the plant is weak and the environment conducive of fungal growth, The rust can spread rapidly throughout the plant and to other plants. When the fungus is vigorous like that, it can weaken and kill nepenthes by significantly reducing the photosynthetic area of the leaves, and sapping the plant's energy reserves to feed its own growth.
Personally, I've had total success with curing my infected plants by treating them with the antifungal agent Myclobutanil. It is a systemic plant fungicide which can be found as an active ingredient in many plant fungus-cure products
People often (and erroneously) call this "rust" but it is unrelated to true Rust fungus. It's a fungal infection called Cercospora, and it is a naturally-occurring pathogen that many (most?) species of Nepenthes experience in their native habitats. Healthy Nepenthes growing in proper conditions can and do experience some degree of Cercospora infection in their lifetimes, but unless they are badly stressed then it will cause only cosmetic damage. Making sure the plants are experiencing an optimal range of temperatures and at least a twenty degree differential between night and day will help avoid catastrophic rampant infections. (Warm nights = stressed Nepenthes, and those stressors will breed rampant infections) But you haven't identified the type of Nepenthes that is, nor have you told us much about the growing conditions and temperatures it experiences.
PS: if the leaf markings become extensive and spread a lot, you can use a fungicide containing Propiconazole to suppress the pathogen.
Last edited by Whimgrinder; 03-26-2015 at 08:55 PM.