Roger of Roles, Rules and Rolls has been creating spell descriptions that fit on a single card, and his most recent post discusses Unseen Servant. I've always kind of liked the basic idea of the spell, but haven't said much about it, since it's AD&D, not OD&D. But it could be translated into OD&D...
The problem, as Roger sees it, is that players will always try to push the limits of what's supposed to be a very simple spell. I don't think anyone's expressed exactly why this is a problem, but I have a suggestion: it's because this behavior is simultaneously against the spirit of the spell and in the spirit of the game. In D&D, you're supposed to be clever, figuring out new ways to use simple tools. But you aren't supposed to do so by adding new capabilities to a tool. With mundane tools, it's easy to make a decision whether a tool could possibly be used in the way described. Throw flour in the air to help spot an invisible entity? OK, makes sense. Throw flour and water in the air simultaneously to make a gooey cloud? That doesn't sound like something that could happen.
With made-up stuff like spells, it gets harder to judge what a spell effect ought to be able to do, unless the effect is clearly described and related to something mundane. The Unseen Servant was meant to just be a simple way to perform menial labor without the caster needing to do the labor himself; it wasn't meant to step on the toes of any higher-level spell, like Invisible Stalker, Wizard Eye, or Telekinesis. But calling it an "unseen servant" or "invisible valet" suggests that it's an independent being, which makes players naturally wonder what that being's abilities are. Can it see? Can it communicate by knocking? How intelligent is it? Can it pick a lock? The spell description makes things worse by confusingly describing the effect as both a servant/valet and as a force, presumably an unintelligent one.
What I would do is describe the effect differently. Perhaps even give it a different name, like Phantom Limb, to help re-define the spell concept. The spell allows multiple, simple actions with a minimum of concentration, as if the caster had an extra invisible, mittened hand that can move independent of the caster's body. The hand is a concept, not an entity; it doesn't sense anything and can't think. The hand can perform actions on objects the caster can't currently see, if the objects are in range and in the same place and condition the caster left them. So, the phantom limb can try to open a door around a corner, as long as the caster has seen the area around the corner before, but if the door is blocked, the caster would have no idea that the door was not successfully opened. The hand can't answer questions the caster doesn't already know the answers to, since it's just an extension of the caster -- so no scouting around corners, but it could be used to send a secret signal to an ally.