The Gemara (Menachos 36b) writes:
אמר ר' אלעזר כל המניח תפילין אחר שקיעת החמה עובר בעשה ור' יוחנן אמר עובר בלאו
R’ Elazar said: whoever places Tefillin after sunset violates a positive commandment. And R’ Yochanan said: he is in violation of a negative commandment.
The Halacha follows R’ Yochanan (Rambam, Hil. Tefillin 4:10-11).
Why should he be in violation already from sunset?
Since it’s a doubt whether it’s day or night between sunset and Tzeis, there seems to be two conflicting mitzvos here: the mitzvah to place Tefillin by day, and since we’re stringent by a doubt of Torah law, one should wear them; and the mitzvah not to place Tefillin by night, and since we’re stringent by a doubt of Torah law, one should not wear them.
But there’s a concept called אתי עשה ודחי לא תעשה, that when a positive commandment and a negative commandment are in conflict, one should do the positive (see further here). Why don’t we extend this principle to here, and one should potentially fulfill the mitzvah after sunset, against the potential prohibition of wearing them at night? Why doesn’t a doubtful positive commandment override a doubtful negative commandment, the same way that a certain positive commandment overrides a certain negative commandment?