Had a filling done on a molar, tooth started bleeding , dentist said he wasnt deep enough to hit the root and some gum tissue had grown into the tooth,have you heard of this?

Not uncommon for a large carious lesion close to the gingival margin. It was the gingiva that was bleeding, not the tooth.

Since you had such a large hole in your tooth, that was level to your gums, it is natural for the gum tissue to grow into the hole. If you had a full tooth structure the gums would be located at the bottom of the tooth near the roots, look at your other teeth. Bleeding is from the inflamed gum tissue.