I almost never screw up a testcase. Almost. But, I screwed one up, on XBL, and sent a bunch of Mozilla VIPs on a wild-goose chase.
That hurts. Particularly since I take pride in making damn sure that I have a valid bug to file before I file one.
Even more embarrassing, I filed bug 117469 ages ago for almost exactly the same kind of bug, but which at the time was causing a crash. (Bug 231396 is the bug where I fouled up, and the similarities between the two bugs are startling…)