My most recent case of browser incompatibility involved a client who does nothing but affiliates.
One day he sends me this email asking me to update a few of his sites with some new videos one of his affiliate accounts came up with.
Once the coding was done I ran the gambit .. Opera, Internet Explorer, Firefox, Chrome, Safari .. well, you know the drill ..
Anyway, the vids worked best in Internet Explorer and Safari .. dragged in Opera, and to my amazement .. choked in Firefox. It appeared that Firefox doesn't do well with "object Moz URLProperty". ..
Most of his visitors use Safari and Internet Explorer anyway, so he wasn't real concerned with my wanting to get after a workaround for this .. I did eventually resolve the issue with Firefox. I'm still surprised that Firefox did what it did.