I had changed a type within a large unwieldy data flow from DT_R4 (float) to DT_R8 (double), since the smaller data type was causing some weirdness around the least significant figures on some option prices. This seemed to work in testing, but I got a cryptic error about a conditional operation failing during a derived column transformation when I deployed it to Production. I opened the package, added an error redirection for the component and a data viewer, and sure enough, it was failing on a transformation that utilized some of the columns I had modified.
I couldn't find any reason why the conditional would fail, since it was faithfully spitting out BOOLs, but the resulting transformation split the float into Integer and Decimal by casting the value into a WSTR, 20 and then finding the decimal point. After trying a dozen other things, I tried adjusting the size of the WSTR cast to a WSTR, 40. That worked.
Note that the values that were failing would not even come close to 40 characters (ex: 110.68), but apparently the possibility of running into an overflow breaks something in the SSIS runtime.
So, a word to the wise: if casting from float (DT_R4 or DT_R8) to Unicode string (WSTR), make sure you leave enough room to cast any possible value.