Pipelined, Superscalar, and OOO Execution Problems

Some instructions interfere with the parallel execution and require instructions to be serialized, e.g. floating point mode change instruction on x86
Data dependencies may cause stalls
Branches can influence which instruction executed
The information about branch may not always be available
Limits on instruction combinations, e.g. Pentium II/III instruction decoder

Some instructions interfere with the parallel execution and require instructions to be serialized, e.g. floating point mode change instruction on x86