Delaminated 3/8" Sheathing
This is an unusable plywood substrate. If you have a "wavey" appearance, there's a good chance this is what your shingles are nailed to. When we replace a roof, the plywood can come apart with the shingle removal. Most times, you can see that the plywood is delaminated and needing to be replaced. However, that is not always the case. Looking for this is something we do on every pre 1985 home.
Overly gapped 1"x board
Most manufacturers will not provide a shingle warranty if shingles are nailed directly over this. Those that do demand a maximum 1/4" gap to be approved. We also check for this prior to installing a roof. We never want to be the cause of a cancelled warranty!