Hello,
to keep it short:
1) Start building a factory
2) Assist the builder with an engineer and before he arrives, pause him
3) When the engie arrives, he starts ''building the factory'', but actually he isnt building the factory, hes just consuming resources
proof and bit of testing in replay below
- works on all factions/factories
- doesn't work when the factory is finished, so it's pretty rare, but it lost a game for me already :S