Bal Tosif — Eruvin 96a
תלמוד בבלי מסכת עירובין דף צו/א
והכא לעבור משום בל תוסיף קמיפלגי דתנא קמא סבר לעבור משום בל תוסיף לא בעי כוונה ורבן גמליאל סבר לעבור משום בל תוסיף בעי כוונה... ועוד הישן בשמיני בסוכה ילקה
The Rogatchover (Miluim 3c; Mahadura Tinyana 51b) inquires: Why does the Gemara here pose its question concerning bal tosif on the basis of the case of a person who sleeps in a Sukkah on Shemini Atzeres — who may or may not intend to fulfill the mitzvah. It would seem that a much more direct question might be asked: What is the status of a person (outside the Beis HaMikdash who took up his lulav and made a berachah upon it the second day (prior to R' Yochanan ben Zacci's decree that the lulav should be taken up. This person clearly intended to fulfill the mitzvah beyond its allotted time. Clearly, in the latter case there is no problem of bal tosif — why not?
Explains the Rogatchover: Bal tosif teaches us that when a mitzvah involves a specific number (viz of things or toys), that number cannot be changed. It is for this reason that adding an additional bayis to Tellifin is forbidden by bal tosif. For this to be the case, there must have been an original numbering sequence. But in the mitzvah of lulav, since the mitzvah is already fulfilled by picking it up once, there is no numbering sequence that has to be followed, (one does not constitute a sequence) and hence no way in which bal tosif can be applied.