A person who sets out of the place where he's establishing his eruv, and he's somehow detained or delayed, he can still continue to that place, but nobody else can rely on that eruv. This seems to be the diminishment of eruv techumin - meaning, all you need to do is set out on your way, and not even get there, to establish residence. And then just one step seems to be enough... Though this is more demonstration of intent than many other mitzvot require. Now, if you've set foot out of techum, without an eruv, you may have an amah of wiggle room, but beyond that machloket, you can't go back in. Plus: The third mishnah on the daf (of four), and the shortest piece of Gemara on a mishnah that we have ever seen. Specifically, on how inexact the surveyors who demarcate the location of techum Shabbat set that line, to allow a traveller who makes it close to the city, but not to the gates, can still get there and not spend Shabbat outside of the city. A fair application of how generous the setting of limits can be.