in the GUI, that dark synchronise screen, if "Progress increase per hour" goes negative, then "Estimated time left until synced" says "%n left", not integers, but '%n'. I was expecting a meaningful wording, like "unknown" or "infinity".

can you reproduce the problem?

I get this behavour by suspending a non-synced node for a few hours and then restoring it.

What version of bitcoin-core are you using?

0.15.99

machine:

Atom 330, 2GB RAM, 5400 RPM HDD, GNU/Linux