I used to do a lot of in situ IHC on mouse embryos and had many problems with the pap pen (mainly my Triton x100 used to eat it all away in 5min and my antibodies used to spill...). I tried tween20, still same effect.
I found a new product on the market from a local startup company (invignome) and it worked well for me. The best thing about it is that the pens come in six colors so I used to mark my negative controls with the red marker and that narrowed down the number of "primary antibody, primary antibody... primer... OOPS! - that was(!) the negative control" sort of mistakes.
Just incase you guys suffer from the same issues.