It has happened to me a few times. I've read through a question and answer, misunderstood some key point and accidentally voted the wrong way. Then a minute later I realize what I have done and try and undo my vote only to have it be locked on me. Surely I can't be the only one who has experienced this?
Sure you could argue that I need to read more carefully before I vote, but I think anyone is capable of making the occasional mistake. You can also kind of work around this issue by doing a quick edit to the question or answer that has a locked vote and then correct the vote, that is less then ideal though.
I've also experienced the occasional technical hiccup where my vote is marked on the client but when refreshed is not marked on the server. Attempted to re-vote to correct the state is met with the locked message, which is annoying to say the least.
My suggestion is to remove the vote lock or alternatively, increase the grace period to something more reasonable. While I understand that is an attempt to prevent tactical downvoting, I think the solution could use improvement. How about your votes only get locked on a per question basic if you have a "competing" answer?
Basically, I understand the need to prevent gaming, but this is one of those anti-gaming countermeasures that decreases the usability of the site is a highly visible way.