most times I've encountered that problem, I've turned the steering wheel to the locked position when turning the car off and forget that I need to turn the wheel a little to get the ignition to release. Otherwise, I've never dealt with it.
i had that problem this month.....to fix it i just sprayed wd40 on my key and put it in and repeated numerous times. You will notice that when u take the key out each time it is covered with dirt, be sure to wipe off the key each time. Keep repeating the wd40 and your problem will be fixed. This problem occurs because the ignition hole is dirty, once you clean out most of the dirt you wont have that problem anymore.
It's not a sticky switch or any thing like that. You Ignition Switch is going bad. Sometimes it will flare up and be a total headache, and sometimes it will work perfectly. It will get worse as time goes on. It is a pretty simple fix really. I've 4 or 5 on a few different Legends. That will fix your problem. All that WD40 or graphite crap is just a tiny, short term, fix and won't last very long. Here's a link to the part.