In the recent div3 contest, I submitted a code which passed the pretest during the contest but gave runtime error at the later checking. The strange thing is that when I submitted the same code again, the error was just gone

Contest Submission: 324871292
Later Submission: 325107819

There are no buffer overflows, all string accesses are bounds checked so this seems like a toolchain issue to me, even though how improbable that sounds