intnum() unfortunate events

[Ilya Zakharevich <nospam-abuse@ilyaz.org>]

? f(N)=intnum(X=-N,N, erfc(X)*X) + erfc(-N)*N^2/2 - erfc(N)*N^2/2
? for(P=1,15, default(realprecision,10*P); print(10*P, " => ", f(100)))
10 => 3.662944319
20 => 0.92567475529080305843
30 => 0.925674755290803058426201666568
40 => 0.9256747552908030584262016665683056207657
50 => 0.50162179820406624614579761058107351307426697331954
60 => 0.501621798204066246145797610581073513074266973319535028128244
70 => 0.5016217982040662461457976105810735130742669733195350281282443425757388
80 => 0.50162179820406624614579761058107351307426697331953502812824434257573883004049703
90 => 0.501621798204066246145797610581073513074266973319535028128244342575738830040497026307707323
100 => 0.5016217982040662461457976105810735130742669733195350281282443425757388300404970263077073230655994389
110 => 0.50162179820406624614579761058107351307426697331953502812824434257573883004049702630770732306559943887226749891
120 => 0.500000000000693926682552593536800828765544365001167113267488795418799719804144680623683190758564267373899015537770530587
130 => 0.5000000000006939266825525935368008287655443650011671132674887954187997198041446806236831907585642673738990155377705305876316510225
140 => 0.50000000000069392668255259353680082876554436500116711326748879541879971980414468062368319075856426737389901553777053058763165102256015532935
150 => 0.500000000000693926682552593536800828765544365001167113267488795418799719804144680623683190758564267373899015537770530587631651022560155329352695624492

Note that this is a very different situation than spikes in the
integrant (which ARE very hard to detect).  THIS ("jumps" of a
function sitting between grid points) would be detected by ANY minor
shift in the integration grid...

Any thoughts?
Ilya