There are a few UAL instructions that can assemble as either ARM code or Thumb code, but not both. You can identify these instructions in the source code using the following command line option when invoking the assembler:

armasm --diag_warning 1812

It warns for any instruction that cannot be assembled in the instruction set opposite to the current one. This is only a hint, and other factors, like relocation availability or target distance might affect the accuracy of the message.