if the plant auto was accidently crossed with a non auto you should get a mixture of both the parents. Meaning some seeds will auto, and some seeds will not. Then depending on the genetics of the non auto, you might see 15-25% of the seeds having a mixture of unstable genetics. Most of the seeds will either be auto or no auto, with the small percentage of the seeds being a mixture. If you found the a good stable plant that has a mix, you could backcross it with the original auto mother. There is really no point though because a lot of the time you will lose thc. You are better off taking two very different fire autos like one sativa and one indica. The farther apart the two strain are the more hybrid vigor. Your best bet is crossing two very stable autos and you should come out with a 50/50, 25/75, 75/25, etc mixture in the seeds depending on which plant has the more dominant traits.