I couldn't see a more specific board for this question, so I'm posting this here.
Recently, I was taking some images of a sunset. I was using a Fuji XT1 with Lee 0.9 hard ND grad to begin with, then added a Hi-Tech 0.9 ND full grad which I placed into slot 2. The moment I put the full ND grad into the filter holder a flare appeared. When I took the filter out again, the flare didn't appear. When I took the filter holder off completely, there was no flare visible. This leads me to think that the flare has something to do with the full ND grad.
I've uploaded the photo which is unedited. I didn't really get the time to mess about and take comparison images with and without the filter, but the image I've attached is with both filters. You can see the lens flare just under the lighthouse.
Settings: 10-24 at 16mm, ISO 100, f16, 1.9 seconds.
I'm wondering if certain combinations of filters increase the likelihood of a flare, especially with sunsets. Of course, I'll have to try this out again at some point, but I've never come across this before.
I'm interested to read thoughts on this.