These are some of the best known glitches in the Pokemon games Red and Blue. MissingNO and 'M are classified as ??? Pokemon, with the number 000. Here is a picture:

To get these Pokemon:

Talk to the old man from Viridian. Say no to his question.

Go to Cinnabar and surf along the coast.

Also, they both duplicate the sixth item in your bag by a certain number. I don't know which number however. 

Another way to capture a MissingNO is to trade with someone at Cinnibar Island. Then, surf on the coast. The MissingNO will be at Level 80. (Do this if you cannot find a MissingNO by doing the other method (that happens))

NEW UPDATE! Three new types of MissingNOs discovered! Click HERE to read!