Here is a passage from the seder vayyosha‘ of Meshullam bar Qalonymos, as it is printed in the 1832 Rödelheim Maḥzor (and followed in subsequent printings), for the seventh or eighth day of Pesaḥ:
As you can see, in the middle of the third line there is a little cursive letter ḥeth, with an abbreviation sign, indicating that the cantor (חזן) should begin reading here out loud.
If one examines the other sections of this composition, one will see that this printing does not have any explicit indication of where the cantor should begin. Rather, there is a long paragraph, and then a short paragraph, leading into the large letters indicating the first words of the next verse of the Song of the Sea. Presumably, the cantor is expected to begin out loud at the beginning of each short paragraph.
However, in this section, if the cantor begins at the beginning of the short paragraph, the following situation is likely to occur:
The people mumble the entire long paragraph, and the beginning of the short paragraph, but not the quote from the next verse, because they'll want to say the quote together with the cantor. Thus, they will say: נרדם ואבד שאון בוגדיך / ובני חסידיך: "The multitudes of those that betray Thee slumber and perish, / and [also] the children of Thy pious people." That is, both the Egyptians and the Israelites perish!
(The poem is meant to be understood thus: נרדם ואבד שאון בוגדיך. ובני חסידיך נחית בחסדך: "The multitudes of those that betray Thee slumber and perish. [But] Thou hast led, with Thy mercy, the children of Thy pious people.)
This situation bothered R' Ḥayyim Mordecai Margolies, in the late 18th century, so he wrote, in his famous commentary Sha‘aré Teshuva on Shulḥan ‘Arukh Oraḥ Ḥayyim §490:
ויש ליזהר בפייט השירה באחרון של פסח בחרוז נרדם ואבד כו' רגילין ההמון להפסיק בתיבת חסידך ולהמתין בתיבת נחית בחסדך עד שיגמור החזן כמנהגם בשאר החרוזים ומשנה הוא כי כאן צריך לו' בדבור אחד כל החרוז דהיינו גם תיבות נחית בחסדך המקושר למעלה אל ובני זרע חסידיך כמובן:
One should be careful, in the piyyut of the Song of the Sea on the last day of Pesaḥ, in the stanza beginning "nirdam ve-’avad". For the masses are accustomed to stop after the word ḥasidekha, and wait until the cantor finishes, as they do in the other sections. But this is a mistake, for here one needs to say the whole sentence together, that is, one must include the words "Thou hast led with Thy mercy" [from the next verse of the Song of the Sea], which refers to the previous words, "the children of Thy pious people", of course.
R' Wolf Heidenheim, the editor of the Rödelheim Maḥzor, seems to have decided to solve this problem in an aggressive way: He indicated that the cantor should pick up several lines before the end, already at the words שדי המשלם לשונאיו. This way, the people will see the indication ח' before those words, and stop there, and expect the cantor to pick up there; so nobody will stop after the words נרדם ואבד. And thus, Heidenheim marks the cantor's beginning in this section of the composition, alone out of all the twenty sections (and it is in a different place from the implied place for the cantor's beginning in all the other sections).
A single letter. A story.
