If you send a one-float-element list, such as [list 123(, into the right inlet of a [*~], [+~], [-~], etc., you get the following error message:
"error: inlet: expected 'signal' but got 'list'"
Instead, the one-element list should be treated just as a float, since:
1) [*~], [+~] etc. accept floats on its right inlet
2) a one-float-element list is supposed to be equivalent to a float to all effects in PD.
All control objects (including [route] and math operators) handle one-float-element lists as floats, and one-symbol-element lists as symbols.