The gemara says that if one realized he had stone in his lap and later forgot that it was there, and he stood up and hurt someone, he is exempt from having to pay damages because even though he realized it was there originally, since he forgot he is not considered negligent rather it is considered "shogeg". The Nimukei Yosef explains that at the time he placed the stone on his lap he was not negligent since he expected to be careful, and at the time he forgot he is not negligent because forgetting is an o'nes. Based on this the Nimukei Yosef writes in the name of Rama, that one who intentionally pushed off davening until the last hour of z'man tefillah, and then in the last hour forgot to daven, is an o'nes, so he can daven tashlumin to make up for it. However, the Nimukei Yosef writes that for tefillah one is required to be more careful because we find that chazal themselves prohibited certain activities so as not to push off davening because they were concerned that one may forget. Therefore, if one violates this by intentionally pushing it off and then forgetting they are considered negligent and cannot daven tashlumin. The Yam Shel Shlomo (35) disagrees and says that he can daven tashlumin, as the shulchan aruch paskens (108:8).
The machlokes between the Nimukei Yosef and Shulchan Aruch seems to be, when one intentionally violates a geder or s'yag set up by chazal to prevent one from forgetting to daven, is that called negligent or accident. The Nimukei Yosef seems to understand that the s'yag becomes like the mitzvah itself, where violating the s'yag is tantamount to intentionally violating the mitzvah. But the Shulchan Aruch and Yam Shel Shlomo understand that the s'yag does not become part of the mitzvah itself, so violating the s'yag is still not as severe as the violation of the actual mitzvah, and would qualify as an accident.