I got pregnant with DS 9 months after DD's emergency C-section.
My case is a rarity, but my external scar did get inflamed, and started weeping and oozing a bit at around 24-25 weeks. My OB chalked it up to a really lumpy scar from the EMCS, and said she would cut out the old scar and completely redo it with the second section. She did a great job.
I also developed polyhydramnios at about 30 weeks with DS, and that caused me to contract constantly (nonproductively) from 34 weeks until I delivered at 38. I ended up delivering early because she was concerned about me having been contracting so long on my internal scar, that it might be compromised. The pain was pretty unbearable there at the end. When she did the operation and got down to the uterine scar, she said it had actually held together really well-virtually no thinning, and it wasn't compromised in any way.
So really, it was just the external scar that was having problems.