A ticket comes in: order 8812 shows as cancelled, but the customer’s card was charged. You open log search. The checkout service wrote a couple of hundred lines for that request; the payment webhook handler wrote a few dozen; the job that expires unpaid orders wrote a line for every order it looked at that minute. The answer is in there somewhere. Forty minutes later you find it. The expiry job cancelled the order thirty seconds after checkout began, and the payment provider’s webhook landed half a second after that.

Add six lines to the top of a file and ship it. Every stack trace through that file now carries different line numbers, so a tracker that hashes the trace files a bug it has seen a hundred times as a brand-new issue. Meanwhile an older issue titled TimeoutError has collected comments from people who each fixed a different timeout and wondered why it never closed.

Grouping fails in both directions at once. One bug turns into hundreds of issues because the message embeds an order ID, the line numbers moved, or a framework upgrade changed the frames in the middle. Two bugs turn into one because every timeout in the codebase raises the same exception type from the same retry helper.