Bizarre crash involving local symbol and computing basis in parallel

I accidentally made a typo in a random piece of code I wrote, and I can't understand the ensuing crash. To replicate, put this in a file and run from "needs" until the end.

await = method(Dispatch => Thing)
await Thing := identity
await BasicList := L -> apply(L, await)
await Task := await @@ taskResult

async = method(Dispatch => Thing)
async Function := f -> (x -> schedule(f, x))

repeat = (n, R) -> (
    if n == 1 then return n;
    r := {repeat(n//2, R), repeat(n//2, R)};
    basis(10, ideal vars R);
    t) --                                <-- here is my typo!
repeat = async repeat

end--
restart
needs "parallel.m2"

R = ZZ/101[x_0..x_10]
elapsedTime await repeat(allowableThreads*2, R)

Note the typo I made, the t should have been an r. Here is what I got on a 1.24.05 binary:

i1 : needs "parallel.m2"

i2 : 
     R = ZZ/101[x_0..x_10]

o2 = R

o2 : PolynomialRing

i3 : elapsedTime await repeat(allowableThreads*2, R)
 -- 0.38629 seconds elapsed

o3 = t

o3 : Symbol

i4 : /home/linuxbrew/.linuxbrew/share/Macaulay2/Core/expressions.m2:675:47:(1):[21]: error: unknown engine error
/home/linuxbrew/.linuxbrew/share/Macaulay2/Core/expressions.m2:675:47:(1):[21]: error: unknown engine error
/home/linuxbrew/.linuxbrew/share/Macaulay2/Core/expressions.m2:675:47:(1):[21]: error: unknown engine error
/home/linuxbrew/.linuxbrew/share/Macaulay2/Core/basis.m2:262:10:(1):[19]: --back trace--
/home/linuxbrew/.linuxbrew/share/Macaulay2/Core/basis.m2:262:10:(1):[19]: --back trace--
/home/linuxbrew/.linuxbrew/share/Macaulay2/Core/basis.m2:262:10:(1):[19]: --back trace--
/home/linuxbrew/.linuxbrew/share/Macaulay2/Core/methods.m2:625:51:(1):[18]: --back trace--
/home/linuxbrew/.linuxbrew/share/Macaulay2/Core/methods.m2:625:51:(1):[18]: --back trace--
/home/linuxbrew/.linuxbrew/share/Macaulay2/Core/methods.m2:625:51:(1):[18]: --back trace--
/home/linuxbrew/.linuxbrew/share/Macaulay2/Core/methods.m2:639:23:(1):[17]: --back trace--
/home/linuxbrew/.linuxbrew/share/Macaulay2/Core/methods.m2:639:23:(1):[17]: --back trace--
/home/linuxbrew/.linuxbrew/share/Macaulay2/Core/methods.m2:639:23:(1):[17]: --back trace--
/home/linuxbrew/.linuxbrew/share/Macaulay2/Core/methods.m2:638:26:(1):[16]: --back trace--
/home/linuxbrew/.linuxbrew/share/Macaulay2/Core/methods.m2:638:26:(1):[16]: --back trace--
/home/linuxbrew/.linuxbrew/share/Macaulay2/Core/methods.m2:638:26:(1):[16]: --back trace--
/home/linuxbrew/.linuxbrew/share/Macaulay2/Core/option.m2:40:8:(1):[15]: --back trace--
/home/linuxbrew/.linuxbrew/share/Macaulay2/Core/option.m2:40:8:(1):[15]: --back trace--
/home/linuxbrew/.linuxbrew/share/Macaulay2/Core/methods.m2:154:80:(1):[14]: --back trace--
/home/linuxbrew/.linuxbrew/share/Macaulay2/Core/option.m2:40:8:(1):[15]: --back trace--
/home/linuxbrew/.linuxbrew/share/Macaulay2/Core/methods.m2:154:80:(1):[14]: --back trace--
/home/linuxbrew/.linuxbrew/share/Macaulay2/Core/option.m2:40:8:(1):[13]: --back trace--
/home/linuxbrew/.linuxbrew/share/Macaulay2/Core/methods.m2:154:80:(1):[14]: --back trace--
/home/linuxbrew/.linuxbrew/share/Macaulay2/Core/methods.m2:631:83:(1):[12]: --back trace--
/home/linuxbrew/.linuxbrew/share/Macaulay2/Core/option.m2:40:8:(1):[13]: --back trace--
/home/linuxbrew/.linuxbrew/share/Macaulay2/Core/option.m2:40:8:(1):[13]: --back trace--
/home/linuxbrew/.linuxbrew/share/Macaulay2/Core/option.m2:40:8:(1):[11]: --back trace--
/home/linuxbrew/.linuxbrew/share/Macaulay2/Core/methods.m2:631:83:(1):[12]: --back trace--
/home/linuxbrew/.linuxbrew/share/Macaulay2/Core/methods.m2:631:83:(1):[12]: --back trace--
/home/linuxbrew/.linuxbrew/share/Macaulay2/Core/methods.m2:154:80:(1):[10]: --back trace--
/home/linuxbrew/.linuxbrew/share/Macaulay2/Core/option.m2:40:8:(1):[11]: --back trace--
/home/linuxbrew/.linuxbrew/share/Macaulay2/Core/option.m2:40:8:(1):[11]: --back trace--
/home/linuxbrew/.linuxbrew/share/Macaulay2/Core/methods.m2:154:80:(1):[10]: --back trace--
/home/linuxbrew/.linuxbrew/share/Macaulay2/Core/option.m2:40:8:(1):[9]: --back trace--
/home/linuxbrew/.linuxbrew/share/Macaulay2/Core/methods.m2:154:80:(1):[10]: --back trace--
/home/linuxbrew/.linuxbrew/share/Macaulay2/Core/basis.m2:226:10:(1):[8]: --back trace--
/home/linuxbrew/.linuxbrew/share/Macaulay2/Core/option.m2:40:8:(1):[9]: --back trace--
/home/linuxbrew/.linuxbrew/share/Macaulay2/Core/option.m2:40:8:(1):[9]: --back trace--
/home/linuxbrew/.linuxbrew/share/Macaulay2/Core/basis.m2:144:73:(1):[7]: --back trace--
/home/linuxbrew/.linuxbrew/share/Macaulay2/Core/basis.m2:226:10:(1):[8]: --back trace--
/home/linuxbrew/.linuxbrew/share/Macaulay2/Core/basis.m2:226:10:(1):[8]: --back trace--/home/linuxbrew/.linuxbrew/share/Macaulay2/Core/basis.m2:144:73:(1):[7]: --back trace--

/home/linuxbrew/.linuxbrew/share/Macaulay2/Core/methods.m2:154:80:(1):[6]: --back trace--
/home/linuxbrew/.linuxbrew/share/Macaulay2/Core/methods.m2:154:80:(1):[6]: --back trace--
/home/linuxbrew/.linuxbrew/share/Macaulay2/Core/basis.m2:144:73:(1):[7]: --back trace--
/home/linuxbrew/.linuxbrew/share/Macaulay2/Core/option.m2:17:8:(1):[5]: --back trace--
/home/linuxbrew/.linuxbrew/share/Macaulay2/Core/option.m2:17:8:(1):[5]: --back trace--
/home/linuxbrew/.linuxbrew/share/Macaulay2/Core/methods.m2:154:80:(1):[6]: --back trace--
/home/linuxbrew/.linuxbrew/share/Macaulay2/Core/basis.m2:149:72:(1):[4]: --back trace--
/home/linuxbrew/.linuxbrew/share/Macaulay2/Core/basis.m2:149:72:(1):[4]: --back trace--
/home/linuxbrew/.linuxbrew/share/Macaulay2/Core/methods.m2:154:80:(1):[3]: --back trace--
/home/linuxbrew/.linuxbrew/share/Macaulay2/Core/option.m2:17:8:(1):[5]: --back trace--
/home/linuxbrew/.linuxbrew/share/Macaulay2/Core/methods.m2:154:80:(1):[3]: --back trace--
/home/linuxbrew/.linuxbrew/share/Macaulay2/Core/option.m2:17:8:(1):[2]: --back trace--
/home/linuxbrew/.linuxbrew/share/Macaulay2/Core/basis.m2:149:72:(1):[4]: --back trace--
parallel.m2:12:5:(3):[1]: --back trace--
/home/linuxbrew/.linuxbrew/share/Macaulay2/Core/option.m2:17:8:(1):[2]: --back trace--
parallel.m2:10:28:(3): --back trace--
/home/linuxbrew/.linuxbrew/share/Macaulay2/Core/methods.m2:154:80:(1):[3]: --back trace--
parallel.m2:12:5:(3):[1]: --back trace--
parallel.m2:10:28:(3): --back trace--
/home/linuxbrew/.linuxbrew/share/Macaulay2/Core/option.m2:17:8:(1):[2]: --back trace--
parallel.m2:12:5:(3):[1]: --back trace--
parallel.m2:10:28:(3): --back trace--

If I fix the t to r then I don't get this error. It also doesn't happen if the first input to repeat is at most a little above allowableThreads.

What on earth is happening?

Reproduce the crash with the supplied Macaulay2 input and inspect parallel.m2 first, then trace the reported paths in Core/basis.m2 and Core/expressions.m2. Compare runs with t and r, and with different thread counts. Done means explaining the interaction that produces the unknown engine error and establishing a regression test or a narrowly defined fix.