The halacha is that the bread used for eiruv chatzeiros must be a whole loaf. The Rosh (Eiruvin 7:12) writes that if a piece is missing, as long as the missing portion is smaller than the shiur challah that a baker separates (1/48 of the loaf), the loaf is still considered shaleim, complete. The Korban Nesanel comments that this is true only with respect to the loaf needed for an eiruv. However, with respect to lechem mishne on Shabbos, the loaf is not considered shaleim unless it is completely intact (see Sharei Tshuva siman 274). However, the Shmiras Shabbos K’Hilchisa quotes R’ Shlomo Zalman who writes that both by lechem mishne and eiruv the same shiur of 1/48 applies. What is the issue behind the debate? A whole loaf is needed for eiruv is so that the parties joining the eiruv will not come to fight (m’shum eivah – eiruvin 81) over who gave a whole piece and who gave only part. Korban Nesanel focuses on this reason – a loaf missing even 1/48 is incomplete, but will not lead to eivah; it is therefore acceptable for eiruv, but disqualified for lechem mishne. RSZ”A focuses on the takkanah – regardless of the reason, once we know a whole loaf is required, if missing 1/48 was not a whole loaf it would be invalid. The Rosh must mean that missing such a minute amount does not detract from the shaleimus of the loaf, and it is therefore acceptable both for eiruv as well as lechem mishne.
This is a general issue that comes up with respect to many dinei derabbanan (e.g. see Kovetz Shiurim to Beitzah 4): does the reason for a takkanah or gezeirah govern its parameters and applicability, or is the reason just the motivation that led to creating the takkanah, but the law then takes on an independent life of its own?