Details

The issue occurs when an event is emitted, which satisfies the following conditions:

• Ganache's subscribedTopics property contains the event identifier of the event (hash of the event signature)
• Ganache's contractCache contains an entry for the address associated with the event
• the contract in the cache does not contain an event matching the event identifier

Reproduction steps

1. Start Ganache UI with the project loaded
2. Deploy EventEmitter contract with truffle migrate - grab the contract address which will be used in the next step
3. Remove the build project with rm -rf build
4. Comment out EventEmitter contract from DelegateCaller.sol, update the migration script (as per comments in 1_reproduction.js), redeploy with truffle migrate
5. The migration script 1_reproduction.js automatically calls DelegateCaller.delegateCall() function
6. Click Events tab in Ganache-UI

Explanation:

In step 2. subscribedtopics is populated with the Event from EventEmitter. subscribedTopics is additive, so the subscription is not removed when we redeploy without the EventEmitter contract.

In step 5, the event is emitted, and is associated with the DelegateCaller address. The preconditions described above are met, specifically:

• subscribedTopics must include the event identifier
• The contract cache must have an entry for the address raising the event

The decoder returns undefined if no event matching the event identifier is found, triggering the whole kerfuffle.