Fixtures requiring 90°C wiring.
If your house was built before about 1985 (as mine was) your original wiring will be NM rather than the NM-B now being installed. NM stands for Non Metallic and is commonly called Romex after a common brand name. The only significant difference is that NM is rated for a maximum operating temperature of 60°C (140°F) whereas NM-B is rated for 90°C (194°F). NM-B is also a bit smaller which makes it easier to work with. Normally, this temperature difference is insignificant, but there is one exception: light fixtures which are designed to mount against the ceiling or wall, or which are normally wired by bringing the building wire inside the fixture, are often marked: "min 90°C supply conductors".
This is an easy detail to overlook. The solution is to change the supply wire to NM-B if possible. If this is impossible, then a different fixture should be chosen.
Much more information about this issue here.
Ceiling Fan Attachment
When replacing a ceiling light with a ceiling fan, it is important to correctly support the heavier fan with a suitable electrical box attached firmly to the framing of the ceiling. This usually requires removing the lightweight box and replacing it with one approved for the support of a fan.
Where the existing box is located near the building framing, it is sometimes possible to support the fan by screwing it directly into a joist with suitable screws.