Internet Explorer sometimes has a timing problem when displaying map blocks. If you see gray blocks where map parts are missing, I have come up with two possible solutions.
I'm sure there is a better fix out there, one that I can program so the user doesn't have to fool with this, but I haven't found it. One horrible solution is to put in a delay, but it has a major obnoxious properties. The delay length is a function of your firewall, connectivity, etc. So there is no delay that will work in general without very poor behavior for those with low latency.