Any other combination combination works:
zsh + promptline + qterminal
bash + promptline + urxvt
zsh + not promptline + urxvt

The first line suggests urxvt is the problem, the second, zsh and the third that promptline is the problem.

The problem itself / steps to reproduce:

• set up urxvt with zsh and promptline (obviously)
• type something
• result:
  MYPROMPT>WHATITYPED
• hit Ctrl-C and type something
• result:
  MYPROMPT> WHATITYPEDNOW

notice the whitespace between the > and what I typed in the latter example?
This wouldn't be a major issue itself but when being in the state of having no whitespace one can get wired behavior for example like this:

• be in "missing whitespace" mode
• type cd and the beginning of a folder name but only so far as it is still ambiguos.
• hit tab to get auto-completion
• the result will be:
  MYPROMPT>ccd ...

now there's a 'ccd'.
These are only "easily reproducible" examples but it can get really weird when one is using the terminal for a longer time.