Two days on a router that was never broken
A frame pair that was only half attached was quietly changing which model ran. Nothing threw, nothing logged.
Generations kept coming back in a style I had not asked for. I was certain the model router was broken, on the reasonable grounds that the router is the only thing that decides which model runs.
Two days. I read the routing table line by line, printed the resolved model on every path, and every print said the model I expected.
The request carried a frame pair with only the first frame attached. That half attached pair sent the request down a different branch, and the branch picked a different model. Nothing threw, because a missing second frame is legal input. Nothing logged, because as far as the code was concerned nothing had gone wrong.
Two days spent reading the one component that was doing its job correctly.