Some users have had issues with iCal slowing down and crashing. This behavior sometimes is also coupled with other strange behavior such as disappearing GUI controls.
User wil_low writs:
"I've been having trouble with iCal for the past few days. This is what is happening, I open iCal, then it starts to slow down. I click on the Day/Week/Month tabs and it freezes and the tabs disappear. It wont let me print and iCal doesn't let me click on the dates."
This type of behavior might also be coupled with improperly functioning searches, screen refreshes, etc. Additionally, iCal may not completely load calendar contents. This behavior all indicates corruption in the iCal preferences file, and removing the file should clear the problems. This type of behavior can also affect other applications as well, in which case the best thing to do is also to remove the preference file.
Fix: Remove the .plist file The iCal preferences file is located in the /username/Library/Preferences/ folder and is called com.apple.iCal.plist. Quit the iCal application and then Locate this file and remove it. Then start iCal back up and check to see if the problems has cleared.