Originally Posted by HillRider

Unfortunately, I have. I have an S&S coupled travel bike that the headset loosened on while I was on a business trip and I didn't have a headset wrench. I wound up borrowing a 32 mm open end wrench from the plant where I was working to readjust it.

Since then I have always carried the Park travel headset/pedal wrench Tim describes above. It is a neat tool and very light. But, yes you are correct that this is a very rare problem.

However, if the stem is properly installed and tightened, threadless headsets very rarely need adjustment until they are disassembled intentionally so neither type is at a disadvantage here.