Which words dwell in a mental black hole for you? For me, it’s ‘expedient’, which I always have the devil of a job dredging up, having often to go through a rigmarole of ‘Might is right’, Alcibiades, Thucydides, etc. before getting the word. Sometimes it remains fugitive until a further day. This has happened for years. ‘Petulant’ is beginning to go the same way.
I forget “William Shatner” every other time I try to think of his name.
There are others, but I forget. 
Gee, I’d tell you, but I forget what they are…
*someone * was going to say it sooner or later!
For me it’s Leukimia and poppadum. Could they be related somehow? 
I draw a…a…um…I frequently cannot find the word I’m looking for. It makes me crazy.
Idiosyncratic. I can not remember anything else for now.
There were several weeks recently when I positively for the life of me could not remember the name Knute Rockne.
Wasabi. I always want to call it ginseng for some reason. If I need to come up with the word quickly, I fall back on “green stuff”.
I know the difference between peaches and pears, but when I talk about pears, my mouth says “peaches” instead.
I remember another one: cilantro. Most of the time I can’t think of anything except “fresh coriander”, which is technically correct but not what I’m looking for.
I can never remember the difference between a hemophiliac and a hypochondriac. In my defence, both start with h and end with -iac.