A breakpoint instruction is used with external debugging hardware. Various check instructions will test for conditions, trapping if the test fails. One common example is a check against bounds or limits. Most processors have one or more illegal instructions. These are usually instructions that haven't been implemented yet or instructions that have been dropped from a processor line.
Many processors generate a trap or exception upon encountering an illegal instruction.
Some processors will execute illegal instructions, which can lead to undocumented operations. Undocumented operations tend to change from one batch of processors to another and are highly unreliable. Some processors reserve an opcode that is guaranteed to always be illegal and always generate a trap or exception. For those with high speed connections, the very large single file summary is still on line. A web site on dozens of operating systems simply can't be maintained by one person.