I get it. In my current covers band - nothing at all challenging - there are certain songs that I'm not 100% on. Enough to get by without anybody noticing, but there are always a few points where I have to concentrate to get through it.
The other day, for a laugh, I listened to some of my first band's recordings and decided to play along. It was note-perfect, and I haven't heard them in over 25 years.