Back when I was a bright-eyed youth (and easily influenceable), I'd look up to the towers of intellectual might that were my elders and betters (usually co-workers/bosses/mentors and the like). I'd banter with them, trying not to annoy them with too many n00b questions but seeking to gain some of their knowledge through being in their proximity. A number of times the phrase "it's not as simple as that" came up in conversation in relation to my summary of a particular problem or area of technology.
At the time, this made me doubt myself and my ability to learn and understand the problems in question. This fear inhibited my confidence, and ultimately disempowered me.
Now, it just makes me mad when I realise I'd been right all along. Perhaps I simply hadn't expressed myself well enough, but for all those of you out there in a position of higher knowledge, please, never say this to someone who is genuinely trying to acquire clue. That one phrase alone can do far more harm than it seems.