How to succinctly express the phenomenon of hidden/surprise complications in regards of time estimates?

Is there an idiom/phrase for when you tell someone you'll have something done at a given time, but then more complications arise and it extends the time needed? For example, I'm working on a project and everything seems like it's done, and when I test it, something has broken, and now it'll take more time to finish.

Solution 1:

These unforeseen problems are often called glitches.

glitch : a small problem or fault that prevents something from being successful or working as well as it should:

We'd expected a few glitches, but everything's gone remarkably smoothly.

The system has been plagued with glitches ever since its launch.

Cambridge

One can express the possibility of such things arising by saying :

Barring glitches, we'll be done in three days.