Tonight, I needed over half an hour to debug a lengthy dynamic code of mine which worked fine for a long time. Let me share the findings: The following code gives an error in Mathematica 10.0.1, 10.0.2 on Linux and OS X. It does not appear in 10.0.0 as far as I can tell:

Slider[Dynamic[a, (a = #) &], {0, 2 Pi}]

The output looks like this

and the error messages is

The specified setting for the option FEFloatIterator cannot be used.

A possible workaround is to numericalise (N[2Pi]) the range. Michael was so kind to report the bug and it is now CASE:2323886