diff --git a/models/base_model.py b/models/base_model.py
index e4aff7ee321..65bddbb4a4c 100644
--- a/models/base_model.py
+++ b/models/base_model.py
@@ -114,9 +114,9 @@ def setup(self, opt):
 
                 # Wrap networks with DDP after loading
                 if dist.is_initialized():
-                    # Check if using syncbatch normalization for DDP
-                    if self.opt.norm == "syncbatch":
-                        raise ValueError(f"For distributed training, opt.norm must be 'syncbatch' or 'inst', but got '{self.opt.norm}'. " "Please set --norm syncbatch for multi-GPU training.")
+                    # Plain BatchNorm does not synchronize stats across processes.
+                    if self.opt.norm == "batch":
+                        raise ValueError("For distributed training, --norm batch is not supported. Please use --norm syncbatch or --norm instance.")
 
                     net = torch.nn.parallel.DistributedDataParallel(net, device_ids=[self.device.index])
                     # Sync all processes after DDP wrapping
diff --git a/models/networks.py b/models/networks.py
index ec1686f6492..96d53e904ed 100644
--- a/models/networks.py
+++ b/models/networks.py
@@ -67,7 +67,7 @@ def lambda_rule(epoch):
     elif opt.lr_policy == "cosine":
         scheduler = lr_scheduler.CosineAnnealingLR(optimizer, T_max=opt.n_epochs, eta_min=0)
     else:
-        return NotImplementedError("learning rate policy [%s] is not implemented", opt.lr_policy)
+        raise NotImplementedError(f"learning rate policy [{opt.lr_policy}] is not implemented")
     return scheduler
 
 
diff --git a/tests/test_reliability_regressions.py b/tests/test_reliability_regressions.py
new file mode 100644
index 00000000000..538251679fe
--- /dev/null
+++ b/tests/test_reliability_regressions.py
@@ -0,0 +1,87 @@
+from types import SimpleNamespace
+
+import pytest
+import torch
+
+from models import networks
+from models.base_model import BaseModel
+import util.visualizer as visualizer_module
+
+
+def test_get_scheduler_invalid_policy_raises():
+    param = torch.nn.Parameter(torch.tensor(1.0))
+    optimizer = torch.optim.SGD([param], lr=0.01)
+    opt = SimpleNamespace(lr_policy="invalid", epoch_count=1, n_epochs=1, n_epochs_decay=1, lr_decay_iters=1)
+
+    with pytest.raises(NotImplementedError, match=r"learning rate policy \[invalid\] is not implemented"):
+        networks.get_scheduler(optimizer, opt)
+
+
+class DummyModel(BaseModel):
+    @staticmethod
+    def modify_commandline_options(parser, is_train):
+        return parser
+
+    def __init__(self, opt):
+        super().__init__(opt)
+        self.model_names = ["G"]
+        self.netG = torch.nn.Conv2d(3, 3, kernel_size=1)
+
+    def set_input(self, input_data):
+        pass
+
+    def forward(self):
+        pass
+
+    def optimize_parameters(self):
+        pass
+
+
+def test_ddp_rejects_plain_batch_norm(monkeypatch, tmp_path):
+    monkeypatch.setattr("models.base_model.networks.init_net", lambda net, init_type, init_gain: net)
+    monkeypatch.setattr("models.base_model.dist.is_initialized", lambda: True)
+
+    opt = SimpleNamespace(
+        isTrain=True,
+        checkpoints_dir=str(tmp_path),
+        name="ddp_norm_guard",
+        device=torch.device("cpu"),
+        preprocess="resize_and_crop",
+        init_type="normal",
+        init_gain=0.02,
+        continue_train=False,
+        norm="batch",
+        verbose=False,
+    )
+
+    model = DummyModel(opt)
+    with pytest.raises(ValueError, match="--norm batch is not supported"):
+        model.setup(opt)
+
+
+def _make_visualizer_opt(tmp_path, use_wandb):
+    checkpoints_dir = tmp_path / "checkpoints"
+    (checkpoints_dir / "exp").mkdir(parents=True, exist_ok=True)
+    return SimpleNamespace(
+        isTrain=True,
+        no_html=True,
+        display_winsize=256,
+        name="exp",
+        use_wandb=use_wandb,
+        checkpoints_dir=str(checkpoints_dir),
+        wandb_project_name="test",
+    )
+
+
+def test_visualizer_allows_disabled_wandb_when_missing(monkeypatch, tmp_path):
+    monkeypatch.setattr(visualizer_module, "wandb", None)
+    opt = _make_visualizer_opt(tmp_path, use_wandb=False)
+    visualizer = visualizer_module.Visualizer(opt)
+    assert visualizer.use_wandb is False
+
+
+def test_visualizer_raises_when_wandb_requested_but_missing(monkeypatch, tmp_path):
+    monkeypatch.setattr(visualizer_module, "wandb", None)
+    opt = _make_visualizer_opt(tmp_path, use_wandb=True)
+    with pytest.raises(ImportError, match="wandb package cannot be found"):
+        visualizer_module.Visualizer(opt)
diff --git a/util/visualizer.py b/util/visualizer.py
index a3573faec92..559433018b9 100644
--- a/util/visualizer.py
+++ b/util/visualizer.py
@@ -4,10 +4,14 @@
 import time
 from . import util, html
 from pathlib import Path
-import wandb
 import os
 import torch.distributed as dist
 
+try:
+    import wandb
+except ImportError:
+    wandb = None
+
 
 def save_images(webpage, visuals, image_path, aspect_ratio=1.0, width=256):
     """Save images to the disk.
@@ -63,6 +67,8 @@ def __init__(self, opt):
 
         # Initialize wandb if enabled
         if self.use_wandb:
+            if wandb is None:
+                raise ImportError('wandb package cannot be found. Install "wandb" or run without --use_wandb.')
             # Only initialize wandb on main process (rank 0)
             if not dist.is_initialized() or dist.get_rank() == 0:
                 self.wandb_project_name = getattr(opt, "wandb_project_name", "CycleGAN-and-pix2pix")