I noticed that
Reap unpacks packed arrays. I've looked at previous answers but am still an unsure whether this a serious issue.
E.g. consider Ex 47.3 from EIWL:
Reap to get a list of all cases where
#/2 is used in
Nest[If[EvenQ[#], #/2, 3#+1]&, 1000, 20].
One solution is
On["Packing"] Reap @ Nest[If[EvenQ[#], Sow[#]/2, 3#+1]&, 1000, 20]
but one gets multiple (
FromPackedArray) messages about unpacking arrays due, I assume, to the fact that intermediate results are irregular structures.
Since other users don't appear to see this, here's a screenshot:
BTW, since $3x+1$ is always even, a more efficient coding of the Collatz problem is to replace