The condom could slip off. If condom doesn't have a nipple leave a half inch at the tip to catch the cum. Squeeze tip when rolling down condom to keep out air bubbles. Bubbles can break condoms. Add more lube often.
Dry condoms break more easily.
Guys, if you cum in the condom while fucking, hold the condom at the bottom, and pull out right away. Throw condom away and wash up after. If you're afraid of a condom breaking, use two . Put a non-lubricated condom on first so the second one doesn't slip off it. Lube the outside of the second one.