Singularity - S1-E15
Corrected entry: In this episode, when SG-1 are first coming to the planet, you see a billboard. It says the planet's name is Hanka (Rite of Passage also notes this) PX8 987. In Holiday and Fair Game it is called P8X 987. Also, there are windmills in the background. In Rite of Passage, they are gone.
Correction: The reason it was called P8X987 is because the characters can't be expected to remember every name of every planet they've been to exactly. As for the windmills, Niirti had death gliders attack the planet. The windmills could've been destroyed after teal'c and O'neill went through the stargate.
The planet designation is a legitimate error. The issue is that if he look at the billboard in "Singularity" the planet designation is PX8 987, but in "Fair Game" Daniel talks about how Nirrti wiped out everyone on that planet except for one person. However, he refers to it as P8X 987. This is a problem because he flips the X and the 8. It could be a character error, but it's an error.