Why does lowering the MTU from 1500 to 1499 allow me to access most websites?

I had this issue where I was only able to connect to websites like google.com and ibm.com when the mtu was set at 1500, but if I tried to connect to anything else, it would just show a blank page. When the mtu was lowered to 1499, it started working. I am curious as to why this works and if having the mtu set at 1499 could cause problems in the future? I actually don't know much about this, I just heard about it and am looking for a good explanation.

When I get an explanation of why the MTU was dropped by only 1 byte, I will update my question with the explanation.

That may mean some other device upstream from you has a smaller mtu and someone has mis-configured a firewall to block all ICMP preventing MTU discovery for the path.

Many naive network administrators seem to believe that ICMP has no purpose and you can completely block it without any repercussions.

To be more specific, what's happening is that the IP datagrams have the DF (Don't Fragment) flag set in their headers. They then hit a gateway somewhere along the path between you and the destination which has a smaller MTU than the one that the sender used. That gateway needs to fragment the datagrams further, so it sends an ICMP type 4 message, which essentially means, "I need to fragment these packets, but you said not to. So the destination is therefore unreachable."

Those ICMP packets are not received by the original sender because some gateway along the path has been configured (foolishly) to block all ICMP. So the sender doesn't know that his packets didn't get through and sits around waiting... and waiting. Eventually something will time out. Usually it's the human that times out first. :)