I get the following error occasionally while in raids, only tested in Wintergrasp so far though. The error is triggered at seemingly a random moment and continues to stack rapidly until a reload.

As you can see I've embedded oUF but I've done it properly and that's never given me issues before. I don't think the error is in my layout, I've updated it according to your example in oUF_Lily.

Message: Interface\AddOns!FreeUI\oUF\ouf.lua:112: attempt to call field '?' (a nil value)
Time: Sat Oct 16 14:52:10 2010
Count: 900
Stack: Interface\AddOns!FreeUI\oUF\ouf.lua:112: in function <Interface\AddOns!FreeUI\oUF\ouf.lua:110>

Locals: self = oUF_FreePartyRaidUnitButton24 {
0 =
MasterLooter = {
}
PARTY_LOOT_METHOD_CHANGED = defined @interface\AddOns!FreeUI\oUF\elements\masterlooter.lua:30
ReadyCheck = {
}
Leader = {
}
UNIT_SPELLCAST_CHANNEL_START = defined @interface\AddOns!FreeUI\oUF\elements\castbar.lua:166
UNIT_SPELLCAST_INTERRUPTIBLE = defined @interface\AddOns!FreeUI\oUF\elements\castbar.lua:100
disallowVehicleSwap = false
Power = {
}
DebuffHighlight = {
}
Health = {
}
RAID_TARGET_UPDATE = defined @interface\AddOns!FreeUI\scripts\unitframes.lua:200
UNIT_SPELLCAST_START = defined @interface\AddOns!FreeUI\oUF\elements\castbar.lua:15
unit = "raid23"
UNIT_SPELLCAST_DELAYED = defined @interface\AddOns!FreeUI\oUF\elements\castbar.lua:128
PARTY_LEADER_CHANGED = defined @interface\AddOns!FreeUI\oUF\elements\leader.lua:13
UNIT_SPELLCAST_STOP = defined @interface\AddOns!FreeUI\oUF\elements\castbar.lua:148
UNIT_SPELLCAST_CHANNEL_STOP = defined @interface\AddOns!FreeUI\oUF\elements\castbar.lua:235
UNIT_FACTION =

{
}
PARTY_MEMBERS_CHANGED = {
}
Threat = {
}
Range = {
}
__tags = {
}
id = "23"
UNIT_SPELLCAST_CHANNEL_UPDATE = defined @interface\AddOns!FreeUI\oUF\elements\castbar.lua:212
UNIT_SPELLCAST_INTERRUPTED = defined @interface\AddOns!FreeUI\oUF\elements\castbar.lua:82
bg = {
}
UNIT_SPELLCAST_NOT_INTERRUPTIBLE = defined @interface\AddOns!FreeUI\oUF\elements\castbar.lua:114
__elements = {
}
colors = {
}
Castbar = {
}
RIcon = {
}
menu = defined @interface\AddOns!FreeUI\scripts\unitframes.lua:26
style = "Free - Party"
UNIT_SPELLCAST_FAILED = defined @interface\AddOns!FreeUI\oUF\elements\castbar.lua:64
}
event = "UNIT_AURA"
(_temporary) = nil
(_temporary) = oUF_FreePartyRaidUnitButton24 {
0 =
MasterLooter = {
}
PARTY_LOOT_METHOD_CHANGED = defined @interface\AddOns!FreeUI\oUF\elements\masterlooter.lua:30
ReadyCheck = {
}
Leader = {
}
UNIT_SPELLCAST_CHANNEL_START = defined @interface\AddOns!FreeUI\oUF\elements\castbar.lua:166
UNIT_SPELLCAST_INTERRUPTIBLE = defined @interface\AddOns!FreeUI\oUF\elements\castbar.lua:100
disallowVehicleSwap = false
Power = {
}
DebuffHighlight = {
}
Health = {
}
RAID_TARGET_UPDATE = defined @interface\AddOns!FreeUI\scripts\unitframes.lua:200
UNIT_SPELLCAST_START = defined @interface\AddOns!FreeUI\oUF\elements\castbar.lua:15
unit = "raid23"
UNIT_SPELLCAST_DELAYED = defined @interface\AddOns!FreeUI\oUF\elements\castbar.lua:128
PARTY_LEADER_CHANGED = defined @interface\AddOns!FreeUI\oUF\elements\leader.lua:13
UNIT_SPELLCAST_STOP = defined @interface\AddOns!FreeUI\oUF\elements\castbar.lua:148
UNIT_SPELLCAST_CHANNEL_STOP = defined @interface\AddOns!FreeUI\oUF\elements\castbar.lua:235
UNIT_FACTION = {
}
PARTY_MEMBERS_CHANGED = {
}
Threat = {
}
Range = {
}
__tags = {
}
id = "23"
UNIT_SPELLCAST_CHANNEL_UPDATE = defined @interface\AddOns!FreeUI\oUF\elements\castbar.lua:212
UNIT_SPELLCAST_INTERRUPTED = defined @interface\AddOns!FreeUI\oUF\elements\castbar.lua:82
bg = {
}
UNIT_SPELLCAST_NOT_INTERRUPTIBLE = defined @interface\AddOns!FreeUI\oUF\elements\castbar.lua:114
__elements = {
}
colors = {
}
Castbar = {

Looks like you have registered an event, but no handler on that event in your layout. I'm not sure if I still rely on that "hole" within the oUF core.. if I'm not I'll add an error on event registration.

It would only highlight the issue earlier however, not fix it for you :).