Intrusive R
I got over inserting an extra R into sherbet years ago, but I can't seem to shake the tendency to add one to frustum. (That's the name for what's left when you remove the pointy end from a cone or pyramid.) I keep wanting to say and write frustrum. The fact that it's related to the word frustrate doesn't help me fight the impulse at all.

I have those same two. Although I still say sherbet as "sherbert", but sorbet as "sorbay." It's not a frequent problem because it's too sweet for me, and I never request it. Frustum still is a struggle, but a rare one, since it almost never comes up in conversation except on the subway.
Speaking of topics mathematical, there is a commonly doubled one in integral as "intergral."
Not to mention its displaced articulation in comfortable "comfterble" -- which is the only pronunciation I'm comfortable with.
I figure the intrusive r makes up for the elusive r of library, forming an erudite fruit or cheese (liberry or librie). I tend toward the cheesy pronunciation, personally, in rapid conversation, but either pairs well with a nice Chardonnay.
And don't get me started on why yarmulke never ever has the r pronounced!

Hm. I've never had a problem (if it is a problem) with any of those words except "comfterble," which was how my mother pronounced it. In junior high I trained myself to pronounce it more "correctly" and (usually) I adhere to that. Sorbet is not a major presence in these parts, but having studied French for many years I would automatically give the word a French-ish pronunciation.
How do you folks deal with "February"?

"Febyuary" and "comfterble"
Remember: Women iron on Wednesdays.