This holds most true for the phrase "I don't know". Not knowing is okay, as long as you're explicit about it. If you merely provide context and some details, that's a start, but you're then expecting your audience to infer that you don't know the next thing - the conclusion, the root cause, the reaction. Your audience may very well assume that you know a lot - you've just told them many details - and therefore that you know the next thing.... but you're holding out on them! From this point things deteriorate quickly. So head it off; say "I don't know".
A (For Once Non-Software) Example:
Over the past weekend, my apartment building lost gas service. No hot water, no heat, etc. This started early Saturday morning. Sunday afternoon a note was slipped under everyone's door. The note explained in great detail that a water main a block away had broken and had leaked into the gas line. So the gas line had to be emptied, dried out, repaired, and checked. After that work was all done, the building gas lines would have to be checked and the system brought back up.
So what was not said?
"I don't know when we will have hot water again."
I should note that building management is more reactionary than proactive about communicating with the residents in general. I should also note that the morning newspaper had an article about the water main break (it was a big one!) and indicated in that article that gas service was likely to be restored Sunday. Because building management didn't say "I don't know", residents were left to assume that hot water would be restored sometime Sunday and that building management was simply not being communicative.
There were a lot of angry residents come Monday morning, and all because "I don't know" wasn't said!