It isn't so much the mistakes that'll bite you when you're solo as it is the need for help with no one around, whether that need flows from a mistake or not. Certainly there are circumstances where a buddy is more a liability than an asset. Instructors frequently dive that way.
But there are "no-mistake" circumstances where solo is a death sentence, and it is this additional risk that solo divers must accept, or they are deluding themselves in their assessment of their safety.
In general, no matter how competent a diver you may be (Wes Skiles is a recent case-in-point), any debilitating event - stroke, heart attack, injury, entrapment, seizure, etc. - while solo diving results in drowning and death, while the same event with a buddy may or may not kill you, because a buddy may be able to transport you to life-saving assistance before you die... when you can not do it alone. To think "it'll never happen to me" is not an acceptable or accurate assessment of this additional risk to solo diving. It's just a part of solo diving and those of us who do choose to dive solo from time to time must consciously accept that risk every time we do it, or we're living in fantasy land.
Rick