Peek-a-boo
Whilst working on a "secret project" recently, I came across an rendering error that manifests itself in IE6. Referred to as the "Peek-a-boo" bug, it's shows itself (or actually, it doesn't) by having your content disappear completely, then mysteriously re-appear on a repaint caused by scrolling or switching back to it from another application.
At first, I was almost convinced it was my graphics drivers, or some odd Windows quirk. However, it's a known issue and fortunately the link above documents it and a good workaround. I chose to use the setting of the line-height attribute to 100% on the parent element which cured it straight off, without affecting the flow of the content itself.