A good trick I learned from another builder.

Whenever gluing critical components (especially where it's hard to see), leave some epoxy in your mixing cup with the stir-stick.

After the allocated cure time, check your sample epoxy to make sure it hardened correctly. Sometimes by incorrectly mixing, or from old epoxy, etc., it just doesn't cure right. I've at least one occasion in which the epoxy in my cup was still gooey the next day. I knew I had to take the joint apart and re-glue. Most parts are obvious if the epoxy doesn't cure. But for some parts, like joining two layers for a firewall, it's not as obvious.