Very strange

• Planned and done:
• Easy half of Micro assignment (first year calculus again!)
• Read International article
• Planned and not done:
• Start math assignment
• Summarise International article
• Not planned, but done:
• Cooked two dinners
• Massive amounts of filing
• Mowed lawns
• Read an entire novel

In my living room there is a central light fitting that holds three bulbs. It's attached to a dimmer switch. For the past year or so it's contained two CFL bulbs and one incandescent bulb (I can't now remember why this is). This morning the incandescent bulb blew. In the time before it was replaced I observed the following phenomena, all of which disappeared when an operative incandescent bulb was installed in the third socket:

• The two CFLs emitted a soft humming sound while switched on
• They spluttered and pulsated wildly when the dimmer was operated
• They flickered gently and continuously while switched off

My questions are, why does this happen with two CFLs in one fitting, and not with any other CFLs in the house? And why on earth should installing a single working incandescent in parallel cause these phenomena to cease?