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I've got a single speed bike with bullhorn handles. When I'm climbing steep hills I pull on the bullhorns quite hard, and once I'm back on a flat I'll occasionally find that the front wheel has become misaligned.

It's a new bike, could this be due to the amount of lubrication on the stem that will fix itself over time?

If not, I presume I could tighten the stem, but would that introduce any new problems?

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That's scary, you shouldn't be able to do that. It sounds as though you can straighten it easily too. If you straddle the front wheel facing backwards can you twist the bars in the fork easily?

If so, definitely tighten it up. Firmly. You should not be able to move that!

Who put the bike together? If it's new, take it back and talk to the owner of the shop - this is something they should be aware of so they can (re)train whoever assembled/serviced the bike. Of course, if it's a big box shop paying schoolchildren $2/bike to take them out of the box and assemble them they won't care, but you shouldn't be buying bikes from them for exactly that reason.

(edit: answer second part of the question) Over-tightening it could break things, yes. But that's normally quite hard to do. If it's a new bike get the shop to do this. If it's your bike it's easy enough to do.

For a modern threadless headset there will usually be two allen key bolts to tighten, and the best solution IMO is the Rolls-Royce one* - use a multi-tool with a short handle, so you don't have enough leverage to overtighten it. Do the bolts up firmly, test by trying to twist the handlebars. If you can, do it up a little more firmly. Then stop, because you don't want to strip the stem and there's probably something else wrong.

For a quill stem there will be a single hex head bolt going down into the headset (rather than across the top). But the procedure is the same, tighten it firmly and test.

In both cases, if that doesn't fix it you should open things up and find out why. But I'll leave that for another question.

  • Henry Royce made each spanner of such a length that when tightened firmly the relevant bolt was correctly torqued. There's a description here in the paragraph beginning "It is worth recounting that the ‘S’ series".
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  • I haven't straddled it facing backwards, but I do fix it by squeezing the wheel with my knees and re-aligning the wheel to match the handlebars. I'll definitely be taking it back to the shop.
    – Robin Ashe
    Commented Jul 3, 2012 at 22:19
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    @RobinAshe -- If you can do that then it's definitely too loose.
    – Daniel R Hicks
    Commented Jul 4, 2012 at 1:30
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I suspect you might have either a loose headset or handlebar stem. I would take it back to where you bought it and describe the problem. If it's recently purchased, they would probably fix it under warranty.

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    From the narrative, it's a loose stem. Loose headset would not cause misalignment (other bad things, yes).
    – Ken Hiatt
    Commented Jul 3, 2012 at 22:21

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