If you assist unfinished building or unit of your opponent with a builder, builder will try to capture it. even if progress bar fills, capturing will not actually happen, and it will keep trying to capture it over and over again.
You can assist engineer with mantis or harbringer and they will assist in capturing if there is nothing to aggro against nearby
mantis / harb will not assist ally engineer capturing, but engineers can assists ally engineers capturing
If you and your opponent queue same building in the same place at the same time you can help to build enemy building.
it can often happen with engineers trying to build mass extractor at the same time... but surely happened once to two acus standing in the open place and randomly deciding that it is a good time to start a PD right in the same place and at the same time https://youtu.be/VsJ3Syb8wW4?t=134