Some users report the appearance of E# when viewing transposed songs that contain accidentals.
OnSong uses the enharmonic preference (sharpness or flatness) to determine the enharmonic preference of transposed chords. For instance, the key of G is a sharp key containing an F#. But if transposition results in an F chord required to be played, OnSong will output this as an E# to maintain the enharmonic preference of the key. In music theory E# and F are basically the same, but we have made updates to OnSong's transposition engine to only use E# in keys that contain a sharp in the written key signature such as C# major.
All that music theory aside, most issues can be resolved by declaring a key for your song. For instance, your song may appear to be in the key of D and contain chords like D and A major. But if the song has an accidental C chord, this could cause E# to appear when the song is transposed to the key of G. The more proper key may be the Key of G which can contain the C chord. Transposing then in the key of C will resolve the E# issue.